To find out why a signature looks broken in Outlook, paste its HTML code into this free checker: it spots hosted, embedded (data:) and attached (cid:) images, SVG, oversized widths, the weight of the code and text that becomes unreadable on a dark background, then links each problem to the guide that explains it. The check runs in your browser; the HTML is sent nowhere.
Check a signature's HTML
This tool needs JavaScript. Your HTML stays on your computer anyway: nothing is sent.
What does the email signature checker look for?
| Check | What the sources say | Source |
|---|---|---|
Embedded image (data:, base64) | Not supported in classic Outlook for Windows; supported in Outlook for Mac, Outlook.com, iOS and Android | caniemail |
Attached image (cid:) | A signature set by an add-in adds the image as an attachment with addFileAttachmentFromBase64Async, then references it with cid: | Microsoft Learn |
| SVG | Not supported in an email signature: use a JPG or a PNG. A linked SVG image is also absent from classic Outlook for Windows, Outlook for Mac and Outlook for iOS | Microsoft Learn, caniemail |
| Code length | A signature set by setSignatureAsync is limited to 30,000 characters | Microsoft Learn |
| Image width | Outlook for Windows can clip images wider than 624 px in the reading pane | Exclaimer |
| Alternative text | All non-text content must have a text alternative | W3C, WCAG 1.1.1 |
| Text contrast | A ratio of at least 4.5:1 between the text and the background | W3C, WCAG 1.4.3 |
prefers-color-scheme | Not supported in classic Outlook for Windows or Outlook for Android; supported in Outlook for Mac, Outlook.com and iOS | caniemail |
light-dark() | Not supported in classic Outlook for Windows, Outlook for Mac, Outlook.com or iOS | caniemail |
“Classic Outlook for Windows” means the desktop application here, not the new Outlook. On top of these checks, the tool flags images without a width in pixels, images that point to a file on your computer or a relative address, and images over http://.
How do I read the result?
- Error: something at least one common email client will not display, such as an SVG image or a
data:image in classic Outlook for Windows. - Warning: a rendering or accessibility risk, such as an image without alternative text, text that is too pale or an image that is too wide.
- Good to know: information, such as the number of hosted images or the support for a CSS property.
Each line links to the guide that explains the fix. For a logo that does not display, see Outlook signature logo not displaying; for mobile and SVG, signature with a logo in Outlook for Android and iPhone. If the signature vanished after switching applications, read signature in the new Outlook, and if it is missing only on replies, signature missing or at the bottom of the thread on reply. For a banner that is too wide, there is the banner resizer.
What does the checker not do?
- It does not reproduce a client's dark mode. The dark preview is only a dark background; the contrast calculation covers the text and background colours written in the signature, as inline styles.
- It does not download your images to weigh them. It counts the weight of the code and of the
data:images it contains. For a banner, the file weight is shown in the resizer. - It does not replace a test send. Send yourself the signature in desktop Outlook, Outlook on the web and a phone before rolling it out.
The preview loads hosted images from their address, as the recipient would; links and scripts are disabled in it. To see what a native Microsoft 365 signature does not solve, read the comparison native Microsoft 365 signature: limits and alternative.
Frequently asked questions
Why does my signature look broken in Outlook?
The most frequent causes are images: an image embedded as base64 that the client does not display, a blocked external image, an unsupported SVG, or an image that is too wide. The checker spots them in the pasted HTML and links each one to the matching guide.
Is my HTML sent to a server?
No. The HTML is checked in your browser. Only the preview loads hosted images, from their own address, as the recipient would; scripts and links are disabled in it.
Why do my data: images not display in Outlook?
According to caniemail, classic Outlook for Windows does not support images embedded as base64 (data:), while Outlook for Mac, Outlook.com, iOS and Android display them. Host the image over https:// or attach it inline (cid:).
How does the checker judge dark mode?
It calculates the contrast ratio between the text and background colours written in the signature, and simulates a dark background for text with no background set. It is not an email client's automatic inversion, which the tool does not reproduce.
What is the maximum length of a signature?
Microsoft limits a signature set by setSignatureAsync, the method of an Outlook add-in, to 30,000 characters. The checker flags longer HTML.
Keep a single signature up to date for every employee. ATOM Signatures sets it directly in Outlook from Microsoft 365: CA$1 per user per month, plus taxes, with a 14-day free trial.
Sources
- Microsoft Learn — Office.Body (SVG, 30,000-character limit): learn.microsoft.com
- Microsoft Learn — Outlook mobile and events (
addFileAttachmentFromBase64Async): learn.microsoft.com - caniemail — base64 images, SVG images,
prefers-color-scheme,light-dark(): caniemail.com - Exclaimer — Seasonal email signatures and marketing campaigns (624 px): exclaimer.com
- W3C — WCAG 1.4.3 Contrast (Minimum) and 1.1.1 Non-text Content: w3.org
Sources consulted on November 2, 2026.