Why Does Your HTML Email Need a Plain Text Part?

Imagine crafting a perfectly designed email — your logo, colors, bold CTA — only to have it fail silently in 40% of inboxes. Not because of spam filters or poor sender reputation. Because the client that opened it? It was set to plain text by default.

That’s how it works for many users: older devices, certain email clients, even security-conscious enterprises disable HTML rendering. Without a plain text fallback, your message doesn’t just look bad — it vanishes.

Adding a plain text part isn’t a formality. It’s a deliverability requirement. For every HTML email you send via SMTP, a plain text alternative is not optional. It's a necessity — both for access and for inbox placement.

Key takeaways

  • HTML emails without plain text fail to render on older or low-resource devices, reducing visibility.
  • Spam filters often flag emails missing a plain text part as suspicious or poorly constructed.
  • Including both HTML and plain text improves deliverability and ensures your message reaches inboxes, even when HTML rendering is blocked.

How to Add a Plain Text Part to HTML Emails in SMTP

You must use a multipart/alternative MIME structure when sending HTML emails via SMTP. Include both a text/html part and a text/plain part. The plain text version should mirror the core message clearly and accurately—no placeholders, no HTML tags, just readable content. This ensures deliverability across all clients, including those that disable HTML by default. For RFC 5322 compliance, validate the full email structure before sending.

Step-by-step: Building a Proper MIME Email

  1. Structure your email as multipart/alternative. This tells email clients to choose between the HTML and plain text versions based on the user’s preferences and capabilities.
  2. Set the Content-Type for the HTML part to text/html. Include proper markup, but keep it lean—no embedded scripts, heavy styles, or large images. Use minimal CSS that’s safe for older email clients.
  3. Add a separate text/plain part with Content-Type: text/plain. This must be a plain, readable version of your message. Do not copy-paste HTML content and strip tags—rewrite it for clarity and conciseness.
  4. Ensure both parts contain the same core message. A user should not miss key information whether they’re viewing HTML or plain text. If your HTML version says “Your order is confirmed,” the plain text version must say the same.
  5. Use unique Content-Id headers if embedding resources like images. This helps clients distinguish linked content from the main body, especially when handling fallbacks.
  6. Validate your email against RFC 5322 standards. Use tools like MxToolbox or check your headers and encoding with RFC 5322. Malformed headers or incorrect encoding can cause rejection at the SMTP level.

Best Practices for Plain Text Accuracy

Plain text is not a fallback—it’s a requirement. Many corporate and government systems filter or block messages without it. Avoid using placeholder text like “[Insert message here]” or “Click here to view in your browser.” Instead, write the plain text version as if it were the only one delivered.

Use consistent formatting: no alignment tricks, no excessive line breaks. Keep lines under 78 characters to prevent wrapping issues. If your HTML version uses bullet points, replace them with asterisks or dashes in plain text.

Test your full email in multiple environments. Tools like MailTester’s Inbox Placement Checker simulate real inboxes and reveal issues before you send. You can also verify your sender reputation using MailTester’s real-time API before sending large volumes.

“Plain text is not a second-class citizen. It’s the baseline for accessibility and deliverability.”

Remember: if you don’t validate your structure, your email may be rejected by strict SMTP servers—even if it contains perfect HTML. Always test before sending.

Code Example: Multipart Alternative in SMTP

You can add a plain text part to HTML emails in SMTP by structuring your message as a Multipart/Alternative MIME block. Include a unique boundary, first a text/plain section, then a text/html section, and close with the boundary. Senders like SendGrid or Mailgun reject messages with syntax errors, so strict adherence to RFC 2046 formatting is essential.

Step-by-step MIME construction

  1. Start with standard SMTP headers: Include From:, To:, and Subject: fields. These are required and must follow RFC 5322 syntax. Without them, the message will be rejected by most SMTP servers.
  2. Define a unique MIME boundary: Use a random string like --boundary123. This separates the parts and must be consistent across the entire message.
  3. Add the plain text part: Set Content-Type: text/plain; charset="UTF-8". Write the plain text version of your email here. This ensures recipients with plain-text-only clients (like some mobile apps or old mail readers) can understand your content.
  4. Add the HTML part: Set Content-Type: text/html; charset="UTF-8". Include the full HTML body. This is rendered by email clients that support HTML, providing visual formatting and links.
  5. Close with the boundary: End with --boundary123-- on its own line. Missing or misformatted boundaries cause parsing failures. Tools like Mailgun and SendGrid will reject malformed MIME.

Why this works

Most modern email clients use the highest-quality available part, but fall back to text/plain if HTML fails to render. This pattern is an industry-standard method for maintaining deliverability and readability. The Multipart/Alternative structure is defined in RFC 2046 and widely supported.

Don’t rely solely on your email software to generate this correctly. Test your final output with real tools like MailTester’s inbox placement test to validate how your message renders across clients and filters.

Remember: a single missing newline, incorrect boundary, or missing charset can trigger blocking. Use a MIME library or validator during development. If you send bulk emails, verify your list with MailTester’s bulk verification to ensure your delivery pipeline stays clean.

Common Mistakes When Adding Text Parts

Sending only HTML emails is a common but critical mistake. Most email clients still require a plain text fallback, and omitting it risks deliverability. Even if your HTML renders perfectly, clients like Apple Mail and older Outlook versions ignore or strip HTML-only content, leaving recipients with blank messages. According to RFC 822, proper email must include both parts to maintain interoperability across systems.

Why Text Parts Matter

  • You’re assuming all clients render HTML correctly—many don’t. Without a plain text part, you risk invisible messages.
  • Using identical HTML and plain text content across both parts flags your email as automated or spam-like—some filters penalize this pattern.
  • Minimal or generic plain text like “See HTML version” or “View in browser” fails when HTML is blocked or disabled, leaving no real content for the user.

Testing Beyond MIME Structure

  • Even with perfect MIME formatting, your email may still be blocked by ISP filters or blacklists due to poor sender reputation.
  • Content filtering can flag emails with little to no plain text content, even if the structure is correct—context matters.
  • Don’t assume compliance means deliverability. A technically correct email can still land in spam or fail to render.
  • Test your message in real environments using tools like MailTester’s inbox placement tester to spot rendering issues before sending to your list.
  • Verify your sender reputation using a service like MailTester’s bulk verification to rule out blocklist issues.
Even a well-structured email with perfect MIME can fail if the domain or IP has a poor track record.

Let’s be honest: most deliverability issues aren’t about formatting alone. They’re about trust. You can craft the perfect HTML and plain text combo, but if your sender reputation is weak, the message still won’t land in the inbox. That’s why you need to test both structure and context. Use real inbox testers—like MailTester’s inbox placement tester—to simulate delivery across major providers and see what recipients actually see.

How MIME Structure Affects Inbox Placement

Proper MIME structure—using multipart/alternative with both HTML and plain text parts—improves inbox placement because spam filters and email clients treat it as a sign of technical competence. Email clients like Gmail, Outlook, and Apple Mail use the plain text version when HTML is blocked, ensuring your message still delivers clearly. This structure also signals good sender hygiene, which supports long-term deliverability and sender reputation.

The Role of Plain Text in Email Rendering

When you send an email, the MIME type multipart/alternative tells the receiving client: “Here’s the same message in two forms—HTML first, plain text second.” This matters because not all clients render HTML by default. Outlook, for example, disables HTML by default for security unless trusted. Gmail strips out some HTML elements unless they’re safe. Apple Mail sometimes rewrites HTML for display. In these cases, the plain text part becomes the fallback—and often the final version the user sees.

Without a plain text version, your message might appear blank, garbled, or blocked. This harms user experience and harms your sender reputation. Spam systems notice high rates of “HTML-only” emails, especially when combined with poor authentication or suspicious content. That’s why major providers like Spamhaus and the IETF consider multipart/alternative a baseline best practice.

Why This Matters for Deliverability and Reputation

MIME structure isn’t just about looks—it’s a signal to email providers. A properly structured message shows you’re following standards. That’s a small but meaningful part of reputation scoring. Providers like Return Path and Mimecast use technical indicators like MIME structure to flag low-quality senders.

For example, if your list contains invalid or catch-all addresses, some may reject your message with a soft bounce. But if your email lacks plain text, the same message might be outright rejected or marked as spam—especially in heavily filtered environments like corporate inboxes. It’s not just a formatting issue; it’s a deliverability risk.

Check your list’s health before sending. Tools like MailTester’s bulk verification detect missing plain text parts, invalid addresses, and other technical red flags that hurt deliverability. You can also test your actual message using inbox placement testing to see how real clients render it.

Let’s be clear: sending HTML-only emails may save a few seconds in development, but it costs you inbox placement, user trust, and long-term reputation. Build the right structure from the start—your inbox placement depends on it.

Testing Your Email’s Deliverability (Including Text Parts)

You can test your email’s deliverability—including both HTML and plain text versions—by sending to real accounts, checking inbox placement through spam filter simulators, and confirming that the plain text part remains intact and readable on all devices and clients. Use tools that analyze headers, spam scores, and content rendering to catch issues before scaling your sends.

Send to Real Inboxes, Not Just Test Addresses

Don’t rely solely on disposable test email addresses. They don’t reflect real-world delivery behavior. Instead, send your email to real inboxes across providers like Gmail, Outlook, and Apple Mail. This tells you whether the HTML and plain text parts render correctly, without being stripped, truncated, or misformatted.

Check if plain text is preserved exactly as intended—no missing lines, garbled punctuation, or excessive whitespace. Some clients, especially older or mobile-only ones, prioritize plain text over HTML. A corrupted or incomplete text version harms accessibility and deliverability.

Use Deliverability Tools to Simulate Real Spam Filters

Spam filters look at more than just content. They analyze headers, sender reputation, and content structure. Tools like those from MxToolbox or Spamhaus provide header analysis and spam score testing. These help you spot red flags—such as missing authentication records, mismatched domains, or overly aggressive HTML formatting—that might block your email.

For a deeper check, use inbox-placement testing tools. They simulate real delivery to major providers and report whether your email lands in the inbox, spam folder, or gets blocked outright. Some tools also show how your content renders across different clients.

MailTester’s Inbox Tester lets you send real messages with full header inspection and deliverability scoring. It validates both the HTML and plain text versions side by side. You can test individual emails or run bulk checks before sending to your list.

For ongoing verification, use MailTester’s real-time API or bulk verification tool to pre-screen your list. This catches invalid, catch-all, or disposable addresses before they impact sender reputation.

Tools to Validate MIME and Content Integrity

You can validate MIME structure and content integrity in HTML emails sent via SMTP by testing the full MIME payload across real email clients. Use MailTester’s inbox-placement tester to simulate delivery, detect structural flaws in your email’s MIME, and verify that plain text parts are properly embedded and rendered. This step prevents bounces, filtering, and broken layouts.

  1. Prepare your complete MIME content — Ensure your email includes both HTML and plain text parts, with proper boundaries and encoding. A missing or malformed plain text part causes delivery issues, especially with older clients or strict spam filters. Use the MIME standard (RFC 2046) as a reference for correct formatting.
  2. Submit the full MIME to MailTester’s verification tool — Paste your full MIME message into the verification tool at MailTester’s bulk list verifier. It checks for syntax errors, missing parts, encoding mismatches, and improper nesting. This catches issues you won’t see in a code editor.
  3. Run inbox-placement tests across real clients — Use MailTester’s inbox placement tester to simulate delivery to Gmail, Outlook, Apple Mail, and others. The test shows how your email renders in real environments, including whether the plain text part is accessible and correctly displayed when HTML is blocked.
  4. Review results and fix issues — MailTester flags problems like missing plain text parts, invalid MIME headers, or content with excessive whitespace that triggers spam filters. Correct these before sending to a live list.

Why Testing Real Clients Matters

Many formatting issues only appear in actual email clients. An email may pass local validation but fail in Gmail due to how it handles MIME boundaries or plain text fallbacks. Testing in real environments — not just theoretical checks — ensures consistency. According to SMTP-Rain, over 60% of email rendering problems stem from incomplete MIME structure, not design.

Automate & Scale with API

For high-volume sending, integrate MailTester’s real-time verification API into your workflow. It validates every email’s MIME structure and plain text inclusion at scale. This prevents bulk send failures and protects sender reputation.

“A well-formed MIME message with a proper plain text fallback is the foundation of inbox delivery.”

Always test before you send. A single malformed email can trigger filtering across multiple platforms. Use MailTester to validate every email’s integrity before it leaves your server.

Why MailTester Helps with Deliverability and Content Quality

You can’t rely on your email server to catch every formatting flaw that harms deliverability. MailTester’s real-time API checks MIME structure, validates HTML and plain text parts, and flags issues like missing text content—before you send. This means fewer bounces, better inbox placement, and higher trust with providers like Gmail and Outlook.

Before Sending: Catch Issues in the Pipeline

SMTP does not test content quality—it only delivers. That’s why sending to a malformed list is like tossing darts blindfolded. MailTester’s verification API checks each email for proper MIME construction, ensuring your HTML email includes a plain text fallback. This is required by industry standards: RFC 2822 and DMARC best practices both stress the need for readable content in multiple formats.

Without plain text, your email may fail compliance checks, trigger spam filters, or get blocked. MailTester detects that risk early. It flags emails that are HTML-only, malformed, or missing a fallback, so you don’t waste sends on addresses that won’t be read—or worse, flagged as spam.

Test How Your Email Feels in Real Inboxes

Even if your email is technically sound, it might still land in spam folders. MailTester’s inbox-placement testing sends your message to real accounts across Gmail, Outlook, Apple Mail, and others—so you see where it lands before your audience does. This isn’t a simulation. It’s testing with actual client behavior, including filtering, preview snippets, and rendering quirks.

When you use MailTester’s inbox tester, you’re not just checking syntax—you’re validating user experience. You can see if your plain text part is cut off, if images fail to load, or if the email gets stripped of critical content. These real-world signals matter more than any deliverability score.

With 98.9% accuracy across millions of checks, MailTester avoids the guesswork. You’re not guessing if an email is valid—your list is verified, your content is checked, and your messages are tested in real environments. For teams using Mailchimp, HubSpot, or SendGrid, seamless integrations make this a natural part of your workflow.

Deliverability isn’t just about reputation—it’s about structure, clarity, and quality. MailTester helps you meet that standard.

Integrating with SendGrid, Mailchimp, and Klaviyo

You can add a plain text part to HTML emails in SMTP when using SendGrid, Mailchimp, or Klaviyo by ensuring your email is sent as a multipart message. These platforms support sending both HTML and plain text versions in a single email. A missing or malformed plain text part can trigger spam filters and hurt your sender reputation. Always include a proper text version for each HTML email.

Configure multipart emails in your platform

  • Use SendGrid’s Mail Send API and include both html and text parts in the payload.
  • In Mailchimp, enable the "HTML and plain text" option when creating a template to ensure both formats are sent.
  • For Klaviyo, use the Email API and supply both html and text fields in the request body.
  • Test your output using tools like RFC 2046 to confirm the proper MIME structure is used.

Verify recipients and test deliverability

  • Before sending, use MailTester’s bulk verification to check for invalid or catch-all addresses that could hurt your reputation.
  • Run inbox placement tests via MailTester’s inbox tester to see how your email lands across Gmail, Outlook, and other providers.
  • Use the Email Verification API in your workflow to validate each address in real time.
  • Review the results — if a recipient shows as “catch-all” or “risky,” avoid including them in campaigns unless you’re certain of their relevance.
Deliverability fails not just from poor content — they also come from technical missteps like missing plain text. Fixing this early saves your sender reputation.

Best Practices for HTML and Plain Text Coexistence

When sending HTML emails, always include a plain text version that mirrors the core message. This ensures readability for users with plain-text clients, email filters, and accessibility tools. Keep the content consistent—don’t leave out key details like links, dates, or unsubscribe instructions. Test both versions separately to catch rendering issues. Use MailTester’s inbox placement tool to simulate real-world delivery and identify content discrepancies before sending.

Align content across both formats

  • Include the same key information in both the HTML and plain text versions—links, CTAs, dates, contact details.
  • Never assume plain text recipients will see the same visual cues. A button in HTML isn’t a button in plain text. Make the link itself self-explanatory: “Click here to view your order” instead of “Click here.”
  • Ensure the plain text version is not just a stripped-down version of HTML but a meaningful, readable alternative. If your HTML copy is 300 words, keep the plain text version at a similar length.
  • If you’re using MailTester’s inbox placement tester, it checks how your email renders across actual client environments—this includes plain text views.

Optimize plain text for clarity and simplicity

  • Use short paragraphs—ideally 1–3 sentences. Long blocks of text in plain text are hard to scan.
  • Avoid formatting like bold, italics, or underlines. These don’t translate well. Instead, use parentheses or all caps for emphasis: “CALL NOW” or “(Important: your account expires Friday).”
  • Keep line lengths under 70 characters for readability on mobile and terminal clients.
  • Don’t use symbols like → or • unless you’re certain they’ll render correctly. Stick to ASCII characters.
  • Use MailTester’s bulk verification to clean your list before sending. A valid list reduces the chance of plain text versions being misrendered due to invalid or catch-all addresses.
“A well-crafted plain text version is not a fallback—it’s a required part of an email’s structure.” start with 100 free verifications.

Frequently asked questionsDoes every email need a plain text part?

Yes—most email clients require it, and missing it harms deliverability and sender reputation.Can I use the same content in both HTML and plain text?

Avoid duplicating content exactly. Use equivalent meaning but different phrasing to reduce spam flags.How do I test if my email has a working plain text part?

Use MailTester’s deliverability test or disable HTML in your email client to preview the plain text version.What happens if I omit the plain text part in SMTP?

Your email may be flagged as spam, rejected by some servers, or fail to render on older devices.Is the plain text part required for transactional emails?

Yes—transactional emails must include both formats to ensure delivery compliance and user experience.Can MailTester check the MIME structure of my email?

Yes—MailTester's inbox-placement and verification tools analyze MIME structure, headers, and content integrity.How many free verifications does MailTester offer?

100 free verifications to start—no expiration on purchased credits.Does MailTester integrate with SendGrid and Mailchimp?

Yes—MailTester integrates with Mailchimp, Klaviyo, HubSpot, and SendGrid for seamless list verification.What is the accuracy of MailTester’s verification?

98.9% accuracy—used by teams to verify large lists and improve deliverability.Can I test deliverability before sending a campaign?

Yes—MailTester’s inbox-placement tests simulate real-world delivery conditions across major clients.Is MIME structure important for SMTP delivery?

Yes—incorrect MIME formatting will trigger rejection, filtering, or delivery failure on many servers.What is multipart/alternative in email?

It’s an email format that sends both HTML and plain text versions so clients can choose the best one to display.SourcesSince May 5, 2025, Microsoft Outlook requires SPF, DKIM, and DMARC from domains sending 5,000+ emails per day, rejecting non-compliant mail outright at the SMTP level with error 550 5.7.515. — Microsoft Outlook requirements (via MailOver bulk-sender requirements guide) (2025)

Keep reading