HTML email checker: will it render in Gmail and Outlook?
In short: Reads the HTML you paste and flags what the big clients drop or mangle: scripts, forms, external stylesheets, flexbox and grid, CSS variables, SVG and WebP images, base64 images, relative image paths, unclosed table tags, missing doctype or charset, and size past Gmail's 102 KB clip. Limit: It reads the code; it doesn't render the email, so a test send is still needed.
Paste your email's HTML and this checker lists what Gmail and Outlook are known to drop or break: scripts, forms, linked stylesheets, flexbox and grid layouts, CSS variables, SVG images, base64 images, relative image paths, unclosed table tags and a missing doctype, plus whether the size passes Gmail's roughly 102 KB clipping point.
Price: this tool is free. Checking a whole email list is from $9/mo, and the first 100 checks are free with Google, no card. See pricing
Sample result
0.7 KB of HTML checked: 3 to fix (a script, an external stylesheet, a relative image) and 10 to check.
- HTML email
- 0.7 KB
- To check
- 10
- Unclosed tags
- <tr>, <td>
What this means for you
Swap flexbox for tables, host the images and inline the CSS before Outlook sees it.
- Scripts, forms and external CSS
- Flexbox, SVG and base64 images
- Unclosed table tags counted
- Size against Gmail's 102 KB clip
- HTML
- Outlook
- Gmail
By SimpleVerifier Team · Last updated · Runs in your browser; nothing you paste is sent to us
Also in: Português (Brasil) · Español
How do I check the list the email goes to?
- Fix the HTML here.
- Upload your send list at /verify. Every address gets a live mailbox check, and valid, risky and invalid download as separate files. The first 100 email checks are free with a Google sign-in, no card.
- Send only to the valid file.
How do I check an HTML email before sending it?
- Copy the HTML from your email builder's code view, or export the .html file.
- Paste it here or upload the file, and press Check the HTML.
- Fix the items marked to fix first (scripts, external CSS, base64 and relative images), then the ones to check.
- Send a test to a Gmail, an Outlook and an Apple Mail address before the real send.
Example
- You enter
- A 0.7 KB HTML email with a linked stylesheet, a flexbox div holding an SVG logo and an 800px hero image at a relative path, a CSS variable, a table with one unclosed row and cell, a form and a script tag, and no doctype, charset or viewport.
- You get
- 3 to fix (the script, the external stylesheet, the relative image src), 10 to check (doctype, charset, viewport, the form, the SVG, styles only in a <style> block, flexbox, the CSS variable, the 800px width, and
<tr>and<td>each opened twice and closed once) and notes for max-width without Outlook code, an image without a width and no title. - Limits
- Runs in your browser, so the HTML is not uploaded and nothing in it is loaded; an uploaded file can be up to 25 MB.
Why does my email look different in Outlook?
Outlook for Windows (the desktop app many offices still run) draws HTML with Microsoft Word's rendering engine, not a browser. Word ignores much of modern CSS: display:flex and grid, max-width on divs, CSS background images and float on most elements. That is why email templates are still built from nested tables with widths set as attributes, and why they carry <!--[if mso]> conditional comments that only Outlook reads.
The new Outlook for Windows and Outlook on the web use a browser engine and behave much more like Gmail, so the problems are mostly in the older desktop app. The checker flags the features that break there and tells you when your file already has Outlook conditional code.
What HTML and CSS does Gmail strip?
Gmail removes scripts, ignores linked stylesheets and @import, and does not display SVG or images written into the HTML as base64 data. It does support a <style> block in the head, with media queries, but Google's CSS support page lists the properties it keeps, and a style block with an error in it can be dropped whole. Inlining the important styles, with the email CSS inliner, keeps the layout when that happens.
Gmail also clips any message larger than about 102 KB, hiding the rest, footer and unsubscribe link included, behind a "View entire message" link. The checker measures the HTML you paste; your sending tool's click tracking makes it longer, so leave some room.
Is this the same as a W3C HTML validator?
No. A W3C validator checks HTML against the web standard and flags things email needs, like bgcolor, align and table layouts, as obsolete. This checker looks only for what email clients drop or mishandle, using the support data published at Can I email. It also counts opened and closed <table>, <tr>, <td>, <div> and <a> tags, because an unclosed cell is the most common way a hand-edited template falls apart in Outlook.
Who uses this tool, and for what?
- Email developers
- Catch a stray flexbox, an unclosed
<td>or a base64 logo before the template goes to QA. - Marketers
- Check a template a designer exported from a web tool before it goes to 50,000 people on Outlook.
- Agencies
- Run a client's hand-edited HTML through one list of checks before sign-off.
- SaaS teams
- Check transactional templates written by developers used to building for browsers.
What can’t this tool tell you?
- It reads the code; it doesn't render the email, so a test send is still needed.
- Support differs by client version; it flags features with known gaps, not every quirk.
- Styles inside a
<style>block are checked for a few features only, not rule by rule. - It can't tell whether the addresses you send to exist.
Common questions
- Does it render the email in Outlook or Gmail?
- No. It reads the code and flags features those clients are documented to drop. Only a test send shows exactly how a client draws the email.
- Can I use flexbox in email?
- Not if your readers use Outlook for Windows, which ignores display:flex and stacks the columns. Gmail, Apple Mail and the new Outlook support it. Tables work everywhere.
- Why are my images not showing in Gmail?
- The usual causes are base64 (data:) images, which Gmail doesn't display, SVG files, which Gmail doesn't display either, and relative paths like images/logo.png that only exist on your computer. The checker flags all three.
- Is my HTML uploaded?
- No. It is read in your browser and nothing is sent anywhere. No image or link in it is loaded.
Sources
A tidy list can still be full of dead addresses.
Extracting, deduplicating and syntax checks all work on the text of an address. None of them can tell whether the mailbox exists. Only the server that hosts it can answer that, through an SMTP check, and that is what the paid product does.
- Upload the cleaned file as it is: CSVs up to 100,000 rows
- A real SMTP mailbox check on every row, with catch-all detection
- Small list? Starter at $9/mo covers 2,500 emails a month, cancel anytime
Plans from $9 a month, cancel anytime, no contract. Sign in with Google and your first 100 email checks are free, no card. The free tools stay free either way.