UnifySig

Guide · HTML

HTML email signature best practices

Most broken signatures break for the same handful of reasons. Email clients are twenty years behind browsers, and classic Outlook renders with Microsoft Word. These are the rules that hold up across all of them, in the order they matter.

Updated 30 September 2026

1. Lay it out with tables, not divs

Flexbox, grid, floats and even max-width are ignored or mangled by classic Outlook. A <table> with cellpadding="0" cellspacing="0" border="0" and one cell per column is the only layout every client agrees on. Nest a second table for a second row of columns rather than relying on wrapping.

2. Put every style inline

Signature editors strip <style> blocks and classes. Each element needs its own style="" attribute: font family, size, colour, line height, padding. Set the font on every <td> and <a>, not just the outer table; Outlook does not inherit reliably.

3. Host images and size them explicitly

  • Host the logo at a public HTTPS address that will exist for years. Gmail drops embedded images entirely.
  • Give every <img> a width and height attribute and the same values in style. Without them Outlook shows the file at its full pixel size.
  • Export at 2× and display at half size so it is sharp on high-density screens: a 240 px file shown at 120 px.
  • Use PNG for logos with transparency, JPEG for photos. No SVG, no WebP; older clients will not render them.
  • Add alt text and style="display:block;border:0" to avoid gaps and borders.

4. Use fonts every computer has

Web fonts do not load in email. Choose from Arial, Helvetica, Verdana, Tahoma, Trebuchet MS or Georgia, and list fallbacks: font-family: Arial, Helvetica, sans-serif. If your brand font matters, the logo image is where it lives.

5. Keep it small

  • Under 600 px wide, so it fits a phone without zooming and a preview pane without scrolling.
  • Text no smaller than 12 px; 11 px only for a legal footer.
  • Whole signature under about 15 KB of HTML. Gmail caps signatures at 10,000 characters.
  • Three to six lines of content. Name, title, company, one or two contact methods, website. Everything else is noise in a reply thread.

7. Expect dark mode

Outlook and Gmail invert or shift colours in dark mode. Use solid dark text on no background rather than white text on a colour block, avoid very light greys (they vanish), and check the logo has transparency or a neutral background so it does not sit in a white box.

Testing checklist

Send a test to yourself and open it in:

  • Classic Outlook on Windows (the strictest renderer)
  • New Outlook or Outlook on the web
  • Gmail on the web, in light and dark mode
  • Apple Mail on a Mac and on an iPhone
  • The Gmail or Outlook app on a phone

Look for: the logo loading, columns staying side by side, no doubled spacing, links the right colour, and nothing wider than the screen. The free generator follows all of the rules above out of the box, if you would rather not hand-write the HTML.

Questions

Should an email signature be an image?

No. A single image cannot be copied, searched or read by screen readers, is blocked by default in many clients, and adds an attachment to every reply. Use text with one logo image.

How do I make the same signature appear on every device?

You cannot from the devices themselves; each client keeps its own. Server-side signature management adds the signature as mail leaves the organisation, so every client, phone and shared mailbox gets the same result.

More guides

Rolling this out to everyone?

UnifySig applies one signature policy to every mailbox in Microsoft 365 and Google Workspace, with names and titles from your directory. Leave your email and we will tell you when it launches.

One email when UnifySig is fully launched. No drip campaign, no sales calls.