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Every email you send through Lettr contains content in one or more formats. The three supported content types are HTML for rich formatting, plain text for universal compatibility, and AMP HTML for interactive experiences. Understanding when and how to use each format directly affects how your emails render, how accessible they are, and how mailbox providers evaluate them. Lettr’s API accepts three corresponding parameters: html, text, and amp_html. At least one of html or text is required for every send.

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HTML Email

HTML is the most common email content format. It allows you to include styling, images, links, buttons, and structured layouts that make emails visually engaging.

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What HTML Email Supports

  • Custom fonts, colors, and spacing
  • Inline images and background images
  • Clickable buttons and hyperlinks
  • Multi-column layouts using tables
  • Open and click tracking via tracking pixels and link wrapping

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Limitations Compared to Web HTML

HTML email is not the same as HTML on the web. Email clients strip or ignore many features that browsers handle without issue:
FeatureWeb HTMLEmail HTML
JavaScriptFully supportedCompletely stripped
External CSS (<link>)SupportedIgnored by most clients
CSS Flexbox / GridSupportedUnreliable — use tables
<video> / <audio>SupportedIgnored by most clients
<form> elementsSupportedStripped by most clients
Media queriesSupportedPartial support
Always use table-based layouts and inline CSS for email HTML. Most email clients do not support modern CSS layout methods like Flexbox or Grid.

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Sending HTML with Lettr

You can send HTML content in two ways: directly via the html parameter, or by using a stored template. 
// Sending raw HTML
await lettr.emails.send({
  from: 'you@example.com',
  to: ['recipient@example.com'],
  subject: 'Your Invoice',
  html: '<html><body><h1>Invoice #1042</h1><p>Amount due: $250.00</p></body></html>',
  text: 'Invoice #1042 — Amount due: $250.00'
});

// Sending via a template
await lettr.emails.send({
  from: 'you@example.com',
  to: ['recipient@example.com'],
  subject: 'Your Invoice',
  template_slug: 'invoice',
  substitution_data: {
    invoice_number: '1042',
    amount: '$250.00'
  }
});

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Well-Structured HTML Email Snippet

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Order Confirmation</title>
</head>
<body style="margin: 0; padding: 0; background-color: #f4f4f4;">
  <table role="presentation" width="100%" cellpadding="0" cellspacing="0">
    <tr>
      <td align="center" style="padding: 40px 0;">
        <table role="presentation" width="600" cellpadding="0" cellspacing="0"
               style="background-color: #ffffff; border-radius: 8px;">
          <tr>
            <td style="padding: 32px; font-family: Arial, sans-serif;">
              <h1 style="margin: 0 0 16px; color: #333333;">
                Hi {{customer_name or "there"}}!
              </h1>
              <p style="margin: 0 0 24px; color: #555555; line-height: 1.5;">
                Your order <strong>#{{order_id}}</strong> has been confirmed.
              </p>
              {{if show_cta}}
              <a href="{{tracking_url}}"
                 style="display: inline-block; padding: 12px 24px;
                        background-color: #0066cc; color: #ffffff;
                        text-decoration: none; border-radius: 4px;">
                View Order
              </a>
              {{end}}
            </td>
          </tr>
        </table>
      </td>
    </tr>
  </table>
</body>
</html>
Lettr’s Topol visual editor generates table-based, inline-styled HTML automatically — you don’t need to write this by hand.

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Plain Text Email

Plain text emails contain no formatting, no images, and no HTML markup. They are raw text delivered exactly as written.

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Why Plain Text Matters

  • Universal compatibility — every email client in existence renders plain text correctly, from modern apps to legacy terminal clients.
  • Accessibility — screen readers handle plain text without any ambiguity. There are no layout quirks or hidden elements to misinterpret.
  • Deliverability — spam filters view emails that include a plain text part more favorably. Sending only HTML without a text alternative is a negative signal.
  • Lightweight — no images to load, no rendering engine needed. Plain text displays instantly.

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What Plain Text Cannot Do

  • No images or embedded media
  • No formatting (bold, italic, colors, fonts)
  • No clickable buttons — links appear as raw URLs
  • No open tracking — open tracking relies on a tracking pixel (<img> tag), which plain text does not support
  • No link click tracking unless the link URL itself is a tracking redirect

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Sending Plain Text with Lettr

await lettr.emails.send({
  from: 'you@example.com',
  to: ['recipient@example.com'],
  subject: 'Your Invoice',
  text: 'Invoice #1042\n\nAmount due: $250.00\n\nView your invoice: https://app.example.com/invoices/1042'
});

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AMP HTML Email

AMP (Accelerated Mobile Pages) for Email brings interactive, app-like experiences directly inside the inbox. Recipients can fill out forms, browse carousels, expand accordions, and take actions without leaving their email client.

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What AMP Email Enables

  • Forms — collect responses, RSVPs, or feedback inline
  • Carousels — swipeable image or product galleries
  • Accordions — expandable content sections
  • Live data — fetch updated content when the email is opened (e.g., real-time order status)
  • Interactive lists — sortable, filterable content

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Important Limitations

ConsiderationDetail
Client supportGmail and Yahoo Mail only. Outlook, Apple Mail, Thunderbird, and most other clients do not render AMP.
Sender registrationYou must register with Google as an approved AMP sender before Gmail will render your AMP content.
ExpirationAMP content expires after 30 days. After that, the HTML fallback is shown.
CachingAMP emails may be cached and served from Google’s servers.
Because AMP support is limited, never send AMP as your only content type. Always include an html fallback so recipients on unsupported clients see a complete email.

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Sending AMP HTML with Lettr

await lettr.emails.send({
  from: 'you@example.com',
  to: ['recipient@example.com'],
  subject: 'RSVP to Our Event',
  amp_html: `<!doctype html>
<html ⚡4email>
<head>
  <meta charset="utf-8">
  <script async src="https://cdn.ampproject.org/v0.js"></script>
  <script async custom-element="amp-form"
          src="https://cdn.ampproject.org/v0/amp-form-0.1.js"></script>
  <style amp4email-boilerplate>body{visibility:hidden}</style>
</head>
<body>
  <h1>You're Invited!</h1>
  <form method="post" action-xhr="https://api.example.com/rsvp">
    <label>Will you attend?</label>
    <select name="response">
      <option value="yes">Yes</option>
      <option value="no">No</option>
    </select>
    <input type="submit" value="Submit RSVP">
  </form>
</body>
</html>`,
  html: '<html><body><h1>You\'re Invited!</h1><p>RSVP at https://example.com/rsvp</p></body></html>',
  text: 'You\'re Invited! RSVP at https://example.com/rsvp'
});

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Multipart Messages — Why Send Both HTML and Text

When you include both html and text in a single API call, Lettr constructs a multipart message. The recipient’s email client decides which version to display based on its capabilities and the user’s preferences.

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Benefits of Multipart Sending

  • Client compatibility — if a client cannot render HTML (or the user has disabled it), the text version is displayed automatically.
  • Deliverability — spam filters evaluate multipart messages more favorably. An HTML-only email without a text alternative is a common spam indicator.
  • Accessibility — screen readers and assistive technologies often work better with the plain text part.
  • User preference — some recipients configure their email client to display plain text by default.

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Example: Sending a Multipart Email

curl -X POST https://app.lettr.com/api/emails \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "from": "you@example.com",
    "to": ["recipient@example.com"],
    "subject": "Your Monthly Statement",
    "html": "<html><body><h1>Monthly Statement</h1><p>Hi {{customer_name or \"Customer\"}}, your balance is <strong>{{balance}}</strong>.</p><a href=\"{{statement_url}}\">View Full Statement</a></body></html>",
    "text": "Monthly Statement\n\nHi {{customer_name or \"Customer\"}}, your balance is {{balance}}.\n\nView your full statement: {{statement_url}}",
    "substitution_data": {
      "customer_name": "Jane",
      "balance": "$1,240.00",
      "statement_url": "https://app.example.com/statements/2026-01"
    }
  }'
When using Lettr templates built in the Topol editor, you can configure a plain text version alongside the HTML version directly in the template settings.

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MIME Structure

Under the hood, email messages follow the MIME (Multipurpose Internet Mail Extensions) standard. When you send both HTML and plain text, Lettr wraps them in a multipart/alternative MIME structure: 
Content-Type: multipart/alternative; boundary="boundary123"

--boundary123
Content-Type: text/plain; charset="UTF-8"

Your plain text content here.

--boundary123
Content-Type: text/html; charset="UTF-8"

<html><body>Your HTML content here.</body></html>

--boundary123--
The email client reads all parts and picks the one it prefers — typically HTML if supported, falling back to plain text. When AMP is included, the structure becomes multipart/alternative with three parts, and clients that support AMP render that version first.

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Content Type Comparison

FeatureHTMLPlain TextAMP HTML
FormattingFull styling (inline CSS, colors, fonts)NoneFull styling + AMP components
ImagesSupportedNot supportedSupported
Link trackingYes (link wrapping)Limited (redirect URLs only)Yes
Open trackingYes (tracking pixel)Not possibleYes
Client supportUniversalUniversalGmail, Yahoo Mail only
Deliverability impactPositive when paired with textPositiveNeutral (requires HTML fallback)
InteractivityStatic onlyStatic onlyForms, carousels, live data
JavaScriptNot supportedNot applicableNot supported (AMP components only)
File sizeModerateSmallModerate to large

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Choosing the Right Approach

Send both HTML and plain text. Transactional emails must reach every recipient reliably. Including a text fallback ensures the message is readable even in the most restrictive environments. The deliverability benefit of multipart messages is especially important for transactional sends that must not land in spam.
await lettr.emails.send({
  from: 'no-reply@example.com',
  to: ['recipient@example.com'],
  subject: 'Password Reset',
  html: '<html><body><p>Click <a href="{{reset_url}}">here</a> to reset your password.</p></body></html>',
  text: 'Reset your password by visiting: {{reset_url}}',
  substitution_data: { reset_url: 'https://app.example.com/reset/abc123' }
});
Send HTML as the primary format with a plain text fallback. Marketing emails rely on visual design to drive engagement, so HTML is essential. Always include a text version for deliverability and for recipients who disable HTML rendering.Use Lettr’s Topol editor to design your HTML visually, and provide a corresponding text version in the template settings.
Only invest in AMP if your audience is primarily on Gmail. AMP email is powerful but niche. Since only Gmail and Yahoo Mail render AMP content, it makes sense only when your recipient base is heavily concentrated on those providers.Always include an HTML fallback (and ideally a text fallback too) so recipients on other clients still see a complete message. Think of AMP as a progressive enhancement, not a replacement.

Sending Content Types

API reference for the html, text, and amp_html parameters.

Templates

Build reusable email templates with the Topol editor and merge tags.

How Email Delivery Works

Understand the full journey of an email from API call to inbox.

Email Rendering Clients

Learn how different email clients render HTML and CSS.