Apple Mail creates some verbose HTML for signatures that when used with an attached image can confuse some Outlook environments and everything will come through as HTML attachments. Ideally link images to web server ones so there is no attachment. Below are instructions and an example.
- Create a signature in Apple Mail > Preferences so a webarchive file is created in /Users/[your-name]/Library/Mail/Signatures
- Create the HTML signature in a text editor and open in Safari
- File > Save As… Web Archive and then navigate to the Signatures folder and click on the most recent signature (xxxx.webarchive) so your file will pick up the same name. OK to replace
- Restart Mail.
- Reassign the default signature in Mail > Preferences