Deliverability

Plain Text vs HTML Email: Deliverability, Tracking and When to Use Each (2026)

You send a plain-text campaign and the dashboard shows zero opens. Nothing is broken. Here is what really changes between designed HTML, plain-looking HTML and true plain text.

By Sangeeth · October 2026 · 21 min read

A SendDoggie customer sent their first plain-text campaign, opened the report the next morning and wrote to ask us why it showed zero opens. Nothing was broken. Open tracking works by loading a tiny image, and a true plain-text email has no way to carry one, so there was nothing to count.

That support ticket sums up the plain text vs HTML question better than most comparison articles do. The format you choose changes how your message renders, what you can measure afterwards, and which technical mistakes you can make. There is also a third option most comparisons skip: HTML built to look like plain text.

This guide covers all three formats, whether plain text actually helps deliverability, how tracking works under the hood, and how to build and test each one.

The short answer: Is plain text or HTML email better?

Neither format wins everywhere. Use HTML email when visuals carry part of the message: ecommerce promotions, product launches, abandoned cart reminders, newsletters and campaigns with several calls to action. Use plain text when the message should read like a direct conversation: founder updates, feedback requests, personal follow-ups and simple service notifications. Use plain-looking HTML when you want the feel of a personal email but still need linked text or open tracking.

1. There are actually three email formats, not two

Most comparisons treat this as HTML versus plain text. Email marketers actually use three distinct approaches, and mixing them up is behind a lot of bad advice.

Designed HTML email

This is what most people picture when they think of email marketing. It supports:

• Product images and branded graphics
• Buttons and styled links
• Multiple content sections
• Brand colors and typography
• Responsive layouts
• Open-tracking pixels

An ecommerce promotion might combine a product photograph, a discount and a prominent Shop Now button. Designed HTML is at its best when the visuals help someone understand the product or the offer. The cost is implementation complexity: email clients support different subsets of HTML and CSS, so a design that looks right in Gmail can break in Outlook.

Plain-looking HTML email

This format gets mistaken for true plain text all the time. It uses minimal styling, often nothing more than a greeting, a few paragraphs, a link and a signature:

Hi Alex,

We just released a simpler way to create email campaigns from product pages.

Would you like me to send you a quick example?

Sam

It reads like a personal email, but underneath it is HTML, so it can still carry an invisible open-tracking image, linked text and tracking redirects. Plenty of email tools label templates like this "plain text" even though they generate HTML. Nothing is wrong with the approach, as long as you know what you are actually sending.

True plain-text email

A true plain-text message has a text/plain body and no HTML alternative. It cannot display:

• Embedded images
• HTML buttons
• Bold or italic formatting
• Hyperlinked words with hidden destinations
• Open-tracking pixels

Links appear as the actual URLs, and the recipient's email application decides how the text is displayed:

Hi Alex,

We just launched our new coffee collection.

You can see it here:

https://example.com/coffee

Let me know what you think.

Sam

2. What actually gets sent: Understanding MIME email formats

Email format is not a visual setting in your editor. It lives in the message's MIME structure. MIME, defined in standards including RFC 2045 and RFC 2046, lets a message contain different content types. Once you can read it, formatting problems, missing tracking data and malformed messages get much easier to diagnose.

Example: A true plain-text email

From: Sam <sam@example.com>
To: alex@example.org
Subject: Quick question about your order
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable

Hi Alex,

You ordered our ceramic pour-over last month.

How is it working out?

If anything is off, just reply.

Sam

The header that matters is:

Content-Type: text/plain; charset=UTF-8

text/plain tells the email client the body is plain text. charset=UTF-8 sets the character encoding so international characters and symbols display correctly. quoted-printable describes how the body is transferred; your sending library encodes characters and line breaks into the right wire format. There is no HTML part.

Example: A multipart HTML email

Most production email systems send HTML and plain-text alternatives together. A simplified structure looks like this:

MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="email-boundary"

--email-boundary
Content-Type: text/plain; charset=UTF-8

Hi Alex,

Our new coffee collection is available.

Visit https://example.com/coffee

--email-boundary
Content-Type: text/html; charset=UTF-8

<html>
  <body>
    <p>Hi Alex,</p>
    <p>Our new coffee collection is available.</p>
    <a href="https://example.com/coffee">
      Explore the collection
    </a>
  </body>
</html>

--email-boundary--

The key header is:

Content-Type: multipart/alternative

It tells the client the message contains alternative representations of substantially the same content. A client that supports HTML shows the HTML version; a text-only client shows the plain-text one.

Why matching content matters

Both versions should carry the same essential information. A common mistake is building a detailed HTML email and leaving the text alternative as:

View this email in your browser.

That is not a useful alternative, and it can trip content-consistency checks in spam filters. For both transactional and marketing email, generate the text version from the same content source as the HTML.

How to inspect an email's actual format

Don't guess the format from how the email looks. In Gmail:

1. Open the received email.
2. Select the three-dot message menu.
3. Click Show original.
4. Inspect the Content-Type headers.
5. Search for text/plain, text/html, and multipart/alternative.

3. Plain text vs HTML email tracking: What can you measure?

Tracking is where the formats differ most in day-to-day use. Some metrics are available whatever the message body looks like; others depend on HTML.

Open tracking requires an image

Open tracking works through a small remote image embedded in the HTML:

<img
  src="https://track.example.com/open/abc123"
  width="1"
  height="1"
  alt=""
/>

When an email client requests that image, the tracking server logs the request. Step by step:

1. The sending platform generates a unique tracking identifier.
2. It inserts an image URL into the HTML.
3. The recipient's email client loads remote images.
4. The tracking endpoint receives a request.
5. The platform records an open event.

Amazon SES supports open tracking through its event-publishing features (see the Amazon SES email metrics documentation). A true plain-text message cannot embed an HTML image, so it cannot be tracked for opens with the conventional pixel method. If a platform quietly adds an HTML part containing a pixel, the message is no longer true plain text.

HTML open tracking is not perfectly accurate either

An open event does not prove that a person read the message. Three things distort it.

Apple Mail Privacy Protection

Apple's Mail Privacy Protection can download remote content privately, regardless of when the recipient actually reads the email, which makes open data unreliable for those recipients. Source: Apple Mail Privacy Protection.

Image blocking

Some email clients and corporate environments block remote images. A recipient can read the entire email without ever generating an open event.

Automated security systems

Some security products inspect incoming mail and fetch remote resources automatically, which registers activity with no human involved.

HTML gives you an approximate open signal. True plain text gives you no pixel-based open signal.

Neither tells you for certain whether someone read and understood the message.

Click tracking works with both formats

Click tracking relies on redirects. Instead of sending the recipient straight to:

https://example.com/products/coffee

the platform swaps the link for something like:

https://links.example.com/r/a8f32b

When someone follows it, the tracking server records the click and redirects to the destination. That works in HTML and plain text alike, but it looks different. In HTML, the tracking URL sits behind descriptive link text:

<a href="https://links.example.com/r/a8f32b">
  View our coffee collection
</a>

In true plain text, the recipient sees the tracking URL itself, which makes a personal message look less natural. It also adds a dependency: if the tracking domain or redirect service fails, the recipient may not reach the destination at all. When a recognizable URL matters more than click analytics, turn click tracking off.

SendDoggie supports plain-text campaigns with configurable click tracking, allowing senders to choose whether tracked links or original URLs are used.

What both formats can measure

Keep in mind that an SMTP acceptance or delivery event does not prove the message reached the Primary inbox. The receiving system can accept a message and still file it in Spam or another folder.

4. Does plain text email have better deliverability than HTML?

This is one of the most persistent claims in email marketing, and the reasoning usually goes like this:

Plain-text emails look personal. Personal emails are less likely to be spam. Therefore, plain-text emails must have better deliverability.

It is too simple to be true.

What Gmail and Yahoo actually require

Google and Yahoo strengthened their sender requirements in 2024. Their published guidance focuses on:

• SPF and DKIM authentication
• DMARC requirements for bulk senders
• Valid sending infrastructure and DNS
• Low spam complaint rates
• Proper message formatting
• Unsubscribe mechanisms for applicable marketing and subscribed messages

Google requires senders of more than 5,000 messages per day to personal Gmail accounts to meet additional bulk-sender requirements, including DMARC and one-click unsubscribe for applicable messages. Neither provider publishes any rule saying plain-text emails get preferential inbox placement over HTML. Sources: Google Email Sender Guidelines and Yahoo Sender Best Practices.

Why badly built HTML can cause problems

HTML opens the door to implementation mistakes that plain text avoids by default. Apache SpamAssassin, an open-source spam filter, includes tests for several of them.

HTML-only messages

The MIME_HTML_ONLY rule flags HTML messages that have no plain-text alternative.

Image-heavy content

Rules such as HTML_IMAGE_ONLY_08 and HTML_IMAGE_RATIO_02 examine image-heavy messages under their respective conditions.

Mismatched alternatives

The MPART_ALT_DIFF rule looks at differences between MIME alternatives.

These are SpamAssassin rules, not a window into Google's or Microsoft's proprietary filtering. Matching one rule does not make a message spam, because filters combine many signals into a score. The lesson still holds: poorly built HTML creates filtering and rendering risks you could have avoided. Source: Apache SpamAssassin test definitions.

What matters more than email format

If a campaign is landing in spam, look at authentication before you look at format.

SPF authentication

SPF declares which sending servers are authorized for a domain. An illustrative record:

example.com. TXT "v=spf1 include:amazonses.com ~all"

This authorizes the mechanisms published by the included domain and soft-fails other senders. It is not a universal AWS SES setup instruction: SES SPF alignment depends on the envelope sender and whether a custom MAIL FROM domain is configured. SPF also limits evaluation to 10 DNS-lookup-causing mechanisms and modifiers, so stacking several email providers into the record without checking the result can cause authentication failures.

DKIM authentication

DKIM adds a cryptographic signature to outgoing messages, and the receiving server verifies it against a public key published in DNS. An SES Easy DKIM setup, for example, uses CNAME records shaped like:

abc123._domainkey.example.com.
  CNAME abc123.dkim.amazonses.com.

The selector and target above are illustrative; the real values come from your sending provider. The usual mistakes are incomplete DNS targets, wrong record names and values mangled while copying them.

DMARC authentication

DMARC ties authentication results to the visible From domain through alignment. A basic monitoring record:

_dmarc.example.com. TXT "v=DMARC1; p=none; rua=mailto:dmarc@example.com"

• v=DMARC1 identifies the protocol.
• p=none requests monitoring without a DMARC-based rejection or quarantine policy.
• rua= specifies where aggregate reports may be sent.

DMARC passes when at least one of SPF or DKIM passes and aligns with the From domain. Once you have reviewed the reports and confirmed every legitimate mail stream is aligned, you can move to quarantine or reject.

What about Gmail's Promotions tab?

A plain-looking email resembles a personal message more than a designed promotion does, but that does not get it into Gmail's Primary tab. Gmail uses many signals to categorize mail and doesn't publish its classification logic, and an email with a product offer can land in Promotions even with no images at all.

Promotions is not Spam. A legitimate marketing email in Promotions is not a deliverability failure, and dressing promotions up as personal correspondence is no substitute for sending relevant content to people who asked for it.

5. When plain-text emails are the better choice

Plain text works best when the message benefits from simplicity and a conversational tone.

Founder and company updates

Say a SaaS founder needs to explain a pricing change. A straightforward note fits better than a promotional template, and the format keeps attention on the explanation:

Subject: A small change to our pricing

Hi Alex,

We're changing how our plans work next month.

The goal is to make pricing simpler for smaller teams.

Your existing plan won't change.

If you have questions, reply here.

Sam

Customer feedback requests

A direct question rarely needs images or buttons. Because the action you want is a reply, true plain text suits it well:

Subject: One question about your last order

Hi Alex,

You bought our ceramic pour-over last month.

We're considering selling replacement filters
in larger packs.

Would that be useful to you?

Just reply yes or no.

Sam
Example Coffee Co.

Re-engagement campaigns

A short note asking an inactive subscriber whether they still want updates often works well in a simple format. It still needs any required unsubscribe mechanism; looking conversational does not remove marketing-email compliance obligations.

Straightforward transactional notifications

Password-reset notices, service alerts and account notifications usually work fine as text. Some transactional emails benefit from HTML buttons, but the essential information should make sense without images. For security-sensitive messages, clarity and recognizable destinations matter more than decoration.

6. When HTML emails are the better choice

HTML earns its complexity when presentation directly helps the reader understand or act.

Ecommerce product promotions

A product photograph shows color, packaging and shape that would take paragraphs to describe. A coffee brand announcing a new roast can show the bag, tasting notes, price and a purchase button in one compact layout, which plain text simply cannot do.

Abandoned cart emails

Abandoned cart reminders work better when they show the exact products the customer left behind. The image gives instant context. Useful elements include:

• Product image and name
• Relevant variant or size
• Price and quantity
• Clear return-to-cart link
• Accurate shipping or discount information

Newsletters with multiple sections

A newsletter with several stories needs visual hierarchy. Headings, dividers, summaries and buttons make it scannable; without them, a long plain-text newsletter gets hard to navigate.

Brand-focused campaigns

For fashion, food, home goods and design-led brands, the visuals are part of the product, and imagery is how they show what makes it distinctive. SendDoggie's email builder supports branded campaign creation, while its Magic Email feature can generate a branded draft from a product URL and a short prompt.

Whatever the tool, start with the content and the campaign goal, then pick the presentation that serves them.

7. How to write a good plain-text marketing email

Plain text takes away almost every visual design option, so the writing, spacing and link placement have to do the work.

Keep paragraphs short

Long unbroken blocks of text are hard to scan on a phone, so aim for two- or three-sentence paragraphs. The internet message format standard, RFC 5322, recommends limiting lines to 78 characters and imposes a maximum of 998 characters per line, excluding the line-ending sequence. Modern sending libraries handle line encoding and wrapping, so you don't need to insert manual breaks every 78 characters.

Put important links on separate lines

Instead of:

See our new products at https://example.com/autumn.

write:

Our new collection is available here:

https://example.com/autumn

This removes ambiguity around trailing punctuation and makes the link easy to spot. Use HTTPS URLs on recognizable domains.

Use plain characters for structure

Plain text has no headings or bold, but it can still be well structured. Skip decorative symbols that make it harder to read.

Three updates this month:

- Free shipping now starts at $40
- Same-day dispatch for orders before 2 PM
- Saved payment methods are now available

Add fallbacks for personalization

A missing first name stands out in a simple email. SendDoggie's merge-tag syntax supports a fallback:

Hi {{first_name|there}},

When the first name is missing, the greeting becomes:

Hi there,

Always test personalization against records with missing or empty fields, and check for whitespace-only names and values with unexpected characters.

Use a real reply address

If you ask for feedback, send from an address that can receive it, not a mailbox nobody monitors:

From: Sam <sam@example.com>
Reply-To: sam@example.com

Keep compliance elements intact

Depending on the applicable law and message type, a plain-text promotional campaign may still need sender identification, a physical mailing address and a working unsubscribe mechanism. US commercial email, for example, falls under CAN-SPAM. Bulk-sender one-click unsubscribe requirements (RFC 8058) depend on message type and sender status, not on whether the body is HTML or plain text.

8. How to build an HTML email that renders correctly

HTML email is not web development. A browser may support a CSS feature that an email client ignores entirely.

Use email-compatible layouts

Table-based layouts are still the norm in production HTML email because they work in the widest range of clients. A minimal example:

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>New collection</title>
</head>
<body style="margin:0;padding:0;">
  <table role="presentation"
         width="100%"
         cellpadding="0"
         cellspacing="0">
    <tr>
      <td align="center" style="padding:24px;">
        <table role="presentation"
               width="100%"
               style="max-width:600px;">
          <tr>
            <td>
              <h1>Our new collection is here</h1>
              <p>Explore the latest products.</p>
              <a href="https://example.com/products">
                View products
              </a>
            </td>
          </tr>
        </table>
      </td>
    </tr>
  </table>
</body>
</html>

This is a simplified skeleton, not a production template. Before using anything like it for real, test responsive behavior, typography, button styling, dark mode and accessibility across the major email clients.

Include a meaningful plain-text alternative

Every HTML email should ship with a text version that keeps the important content, not a different message:

Our new collection is here.

Explore the latest products:

https://example.com/products

Avoid image-only emails

Putting the whole promotion in one big image causes several problems at once. If images are blocked, the reader sees almost nothing. Screen-reader users miss the essential information. Some spam filters also look hard at image-heavy content. Use real HTML text for headlines, descriptions, prices and offers.

Add meaningful image alt text

For a product image, describe what it shows:

<img
  src="https://example.com/coffee-bag.jpg"
  alt="Winter Roast coffee in a kraft paper bag"
  width="300"
  style="max-width:100%;height:auto;"
>

For a purely decorative image, use an empty alt attribute so assistive technology skips it:

<img
  src="https://example.com/divider.png"
  alt=""
>

Source: W3C Web Accessibility Initiative: Images Tutorial.

Watch HTML message size

Gmail is known to clip large HTML messages at roughly 102 KB of message HTML, although the behavior varies. Clipping can hide important content, including the footer, and it breaks open tracking if the pixel sits in the clipped portion. Cut unnecessary markup, deeply nested wrappers and excessive inline styles, and measure the HTML your template builder actually generates, not just the source template.

Test dark mode

Dark mode can change background colors, text colors and how transparent images look. The usual casualties:

• Dark logos disappearing against dark backgrounds
• White text becoming unreadable
• Buttons losing contrast
• Transparent product images blending into the background

Test the actual received message in Gmail, Outlook and Apple Mail, whichever your audience uses.

9. How to test plain text vs HTML email properly

If you want to know which format works better for your audience, run a controlled experiment rather than trusting general claims that one format always wins.

Step 1: Choose a comparable metric

Don't compare HTML open rates against true plain-text open rates; the plain-text side can't be measured with a pixel. Pick metrics both variants report:

• Unique click rate
• Reply rate
• Conversion rate
• Revenue per recipient
• Unsubscribe rate
• Complaint rate, where reporting is available

Decide on one primary success metric before the test starts.

Step 2: Create two versions

Version A is true plain text and Version B is HTML. Keep everything else identical: sender, subject line, offer, essential content and call to action. If you change the subject, length, offer and layout at the same time, you won't know which change caused the result.

Step 3: Randomly divide eligible recipients

Assign recipients to the two groups at random so differences in behavior spread evenly across both. Don't put your most engaged subscribers in one group and the inactive ones in the other.

Step 4: Track conversions with UTM parameters

Plain text:

https://example.com/autumn?utm_source=email&utm_medium=email&utm_campaign=autumn&utm_content=plain

HTML:

https://example.com/autumn?utm_source=email&utm_medium=email&utm_campaign=autumn&utm_content=html

Both URLs go to the same page. The utm_content value tells you which variant the visitor came from.

Step 5: Interpret the results carefully

Suppose the results look like this:

Here HTML produced more clicks and conversions, while plain text produced more replies. Neither is automatically the winner. If the goal was customer feedback, the replies are the valuable outcome; if the goal was sales, conversions and revenue matter more.

Small differences like these may also not be statistically meaningful. On low-volume lists, repeat the test before making large strategy changes on a handful of events.

10. Common mistakes when choosing email formats

Mistake 1: Assuming plain text always avoids spam

Plain text does not make up for poor authentication, a damaged sender reputation or an audience that doesn't want your mail. Fix: Check sending infrastructure, list quality and complaint patterns before changing format.

Mistake 2: Treating plain-looking HTML as true plain text

An email that looks like plain text may still contain HTML, tracking images and redirect links. Fix: Inspect the message's MIME structure.

Mistake 3: Comparing open rates between formats

True plain text has no pixel-based open tracking. Fix: Compare clicks, replies or conversions instead.

Mistake 4: Using unrelated tracking domains

A tracking URL on an unfamiliar domain looks suspicious to recipients. Fix: When you need tracking, use a recognizable, properly configured tracking domain.

Mistake 5: Using public URL shorteners

Public shorteners share their domains with many unrelated senders, which raises reputation and transparency concerns. Fix: Use direct destination URLs or a dedicated branded tracking domain.

Mistake 6: Forgetting personalization fallbacks

A message that opens with Hi , looks careless. Fix: Preview the campaign with contacts that have missing values before sending.

Mistake 7: Sending visual product campaigns without images

Plain text suits a founder's note; it is a poor way to show off a new product collection. Fix: Choose the format based on what recipients need to see to make a decision.

11. Troubleshooting plain-text and HTML email problems

Problem: My plain-text campaign shows zero opens

Likely cause: The message is true plain text, so there is no tracking image. This is exactly the ticket described at the top of this article.

What to do: Judge the campaign on clicks, replies and conversions. If you need an open-based workflow, use HTML and account for the limits of open data.

Problem: Click tracking shows zero even though recipients clicked

Possible causes:

• Click tracking was disabled when the message was sent.
• URLs were not recognized or rewritten.
• Links were malformed.
• Tracking requests were blocked.
• The tracking service did not record the event correctly.

What to do: Send a test message to yourself, inspect the raw URL, follow the redirect and check the tracking endpoint logs.

Problem: A tracking link returns a 404 error

Look for a malformed token, a deleted tracking record, an incorrect route or an expired redirect mapping.

What to do: Test the exact URL from the received message, check server logs and verify the redirect target. With plain-text URLs, also check whether the email client swallowed adjacent punctuation into the link.

Problem: Special characters appear broken

Text like this almost always points to a character-encoding mismatch:

We’re launching tomorrow

What to do: Inspect the original message headers and body encoding, and confirm UTF-8 is used consistently through content creation, storage and message generation.

Problem: Outlook removes line breaks

Some Outlook versions normalize or strip line breaks in plain-text messages.

What to do: Separate paragraphs clearly, keep lines reasonably short and test the received email in the Outlook versions your audience uses.

Problem: HTML email looks broken on mobile

The usual culprits are unsupported CSS, fixed-width containers, large images or broken responsive styles.

What to do: Start from a simple single-column layout with fluid widths, and test in real email clients.

Problem: The campaign lands in Gmail Promotions

Promotions placement is not an error in itself.

What to do: Ask whether the message is genuinely promotional. Removing images or buttons will not force Primary placement.

Problem: Both formats land in Spam

Now you are looking at an infrastructure or reputation problem. Check:

1. SPF, DKIM, and DMARC results in the received message.
2. Sending domain and IP reputation where diagnostic data is available.
3. Spam complaints and bounce patterns.
4. Whether recipients opted in and still expect the messages.
5. Suspicious links, tracking domains, and content.
6. Whether sending volume increased abruptly.

For the full diagnostic process, see Why Are My Emails Going to Spam? 12 Reasons and How to Fix Them.

12. Best practices for plain text and HTML email in 2026

Treat format as a communication decision, not a shortcut around spam filters. For plain text, that means readable paragraphs, recognizable URLs, accurate personalization and a monitored reply address. For HTML, it means real text, matching MIME alternatives, accessible images, responsive layouts and predictable rendering. Whichever you send:

• Authenticate sending domains with SPF, DKIM, and DMARC.
• Use permission-based lists and maintain recipient preferences.
• Make unsubscribe options functional and easy to use.
• Avoid misleading links and unfamiliar redirect domains.
• Test the actual received message rather than relying only on editor previews.
• Evaluate campaign performance using metrics tied to the campaign's objective.
• Monitor bounce and complaint signals.
• Keep transactional and promotional messaging clearly distinguishable.

The best email isn't the most polished or the most stripped-down. It is the one that communicates clearly, works in the recipient's email client and reaches people who want it.

Conclusion

The decision gets easier once you split it into three questions: what does the message need to communicate, what does the recipient need to do, and what do you need to measure?

HTML is the right call when images, structure and buttons improve the experience. True plain text is right when the message is essentially a conversation. Plain-looking HTML sits in between, but don't confuse it with true plain text, especially when you read your reports.

No format guarantees better deliverability. A well-authenticated sender with a healthy reputation, a relevant audience and properly built messages is in a far stronger position than someone switching formats to paper over underlying problems.

Key takeaways

1. There are three practical formats: designed HTML, plain-looking HTML, and true plain text. They are not interchangeable.
2. True plain text cannot track opens using tracking pixels because it cannot embed images.
3. HTML open tracking is approximate. Privacy protections, image blocking, and automated scanning affect accuracy.
4. Click tracking works in both formats, but tracked URLs are visible in true plain-text messages.
5. Plain text does not automatically improve deliverability. Authentication, reputation, complaints, and list quality matter more than format alone.
6. HTML is usually better for visual campaigns, including product launches, abandoned cart reminders, and multi-section newsletters.
7. Plain text is useful for conversational messages, including feedback requests, founder notes, and straightforward follow-ups.
8. Always test using comparable metrics. Clicks, replies, conversions, and revenue are more appropriate than comparing open rates across formats.
9. Build HTML emails with meaningful text alternatives, accessible images, and tested layouts.
10. Choose the format for the message and audience, not as a trick to bypass spam filters.

Sources and technical references

1. RFC 2045 — MIME Part One
2. RFC 2046 — MIME Media Types
3. RFC 5322 — Internet Message Format
4. RFC 8058 — One-Click Unsubscribe
5. Google — Email Sender Guidelines
6. Yahoo — Sender Best Practices
7. Amazon SES — Monitoring Sending Activity
8. Apple — Mail Privacy Protection
9. Apache SpamAssassin
10. W3C — Images Accessibility Tutorial