While SmartrMail lets you add custom HTML blocks to your emails, not all HTML content is supported by email inboxes, even if it displays fine in a web browser.

Important: the following types of HTML are not supported by email inboxes and shouldn't be used in your emails:

The following types are not universally supported — they may work in some inboxes but not others, so use them with caution:

The vast majority of email clients don't support JavaScript. In SmartrMail's HTML blocks specifically, <script> tags aren't stripped — they'll still be included in your email's HTML — but they'll never actually run in any email client. Worse, the script's code can leak out as visible, garbled text in the hidden preview text (preheader) some inboxes show before an email is opened, and in the plain-text version of your email.

Instead: use an animated GIF to make your content more engaging.
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An <iframe> embeds content from one site into another, and the vast majority of email clients don't support it.

Instead: link directly to the content you want to display.
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Most email clients block Flash animations and graphics entirely.

Instead: use an animated GIF to make your content more engaging.
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Some email clients will display form elements (text fields, checkboxes, etc.), but subscribers won't be able to submit them.

Instead: link to a Google Form or a form on your own site.
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Autoplay and click-to-play media won't work unless a subscriber's email client supports HTML5 <video>/<audio> tags — only Apple Mail supports this among major clients, so it's best to avoid embedding media this way.
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Instead: for YouTube or Vimeo content, use SmartrMail's Video block — paste a link, and subscribers whose client supports it will see the real video, with an automatic fallback image for everyone else. For other video sources, link an animated GIF to the video content instead.

With caution — background images aren't universally supported. See our guide on adding a page background for the specific list of which email clients do and don't support them.
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No — no major email client executes JavaScript, regardless of what's in your HTML. If you include a <script> tag, it will never run, and its code may show up as visible text elsewhere in the email (like the preheader), so it's best avoided entirely.
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Use SmartrMail's Video block for YouTube or Vimeo content — it's built specifically to work around this limitation.

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