Apple Mail HTML email signature installation guide

Apple Mail on macOS has no HTML signature field, so the reliable method is to create a blank signature, edit the file Mail saves it in, then lock that file so Mail can’t strip the formatting. These steps work on current macOS.

1Create a blank signature
1. Design and save your signature in Mirosign.
2. In Mail, go to Mail → Settings (press Cmd + ,), then click Signatures.
3. Select your email account in the left column, click + to add a signature, and name it Mirosign signature. Don’t worry about the content — we’ll replace it.
2Quit Mail

Fully quit Apple Mail (Cmd + Q). This is important — Mail overwrites the signature file if it stays open.
3Find the signature file
1. In Finder, choose Go → Go to Folder (Cmd + Shift + G).
2. Paste this path, then press Return:
  ~/Library/Mail/
3. Open the highest-numbered V folder (e.g. V10, V11), then MailData → Signatures.
4. Sort by Date Modified and open the newest .mailsignature file with TextEdit. Do not edit any .plist files.
4Replace the signature code
1. In Mirosign, click Copy HTML.
2. In the file, leave the few header lines at the very top intact. Select everything from <body> to </body> and paste your Mirosign HTML in its place.
3. Save and close the file.
5Lock the file

Right-click the file, choose Get Info, and tick Locked. This stops Mail from rewriting your custom HTML.

Tick Locked in the file’s Get Info panel.
6Check your signature

Reopen Mail and start a new email to check your signature. If it doesn’t appear automatically, pick Mirosign signature from the Signature dropdown in the compose window, or set it as the default for your account in Mail → Settings → Signatures.

Design your email signature