Attention macOS Big Sur (10.16) users!
After your signature is created, you may have to drag/drop it from the 'All Signatures' Signature pane of Mail Preferences into your preferred account (e.g., iCloud, Google) to be able to use it.
If you don't see your signature there, and are running the latest version of the app, the issue may be related to iCloud:
- From System Preferences, choose iCloud
- Ensure that Mail is checked.
- Then, next to iCloud Drive (at the top, likely checked), choose Options...
- Scroll down to find Mail, and ensure it is checked
- Click Done
Finally, please review this article on why images may not display in the Signature panel (but will in the message)!