MacOS – In Yosemite Mail, setting Signature to None does not remove signature like in Mavericks

I had the same problem as you. I did some digging around and found a solution that used to work back in the OSX 10.8 days and stopped working with OSX 10.9. Oddly enough, it works again with OSX 10.10.1 and Mail 8.1 (1993).

There are multiple parts to this - an AppleScript that you compile into an Application, a text file containing your random quotes, two signatures in Mail.app (one is a template, the other is the actual signature for your messages), and the system 'cron' service. (Ideally you should use launchd, but I haven't got that far yet.)

The AppleScript:

if application "Mail" is running then
    set AppleScript's text item delimiters to "~###~"
    set myQuote to some text item of (read "/Users/Steve/.sigs.in")
    tell application "Mail"
       set the content of signature "MySignature" to the content of signature "MyTemplate" & myQuote
    end tell
end if

In my implementation (which you can deduce from the AppleScript):

• My two Mail.app signatures are called "MyTemplate" and "MySignature". "MyTemplate" contains everything that is static - stuff like my email address, etc. "MySignature" is the one that I use with my messages.

• The text file holding my quotes is called ".sigs.in" (so that it doesn't appear in Finder windows) and all the quotes are separated by a line holding only "~###~". This file lives in my home directory.

• My crontab entry consists of

  * * * * * /Users/Steve/bin/MailRandomSig.app/Contents/MacOS/applet 

The compiled AppleScript lives in /Users/Steve/bin (obviously enough), and the "applet" part is the actual compiled binary that we want to run.

I've tested this and it works beautifully. Each minute, cron kicks off my compiled script which updates the signature.

The only "downside" is that if you don't like the quote you have to wait a minute for the script to refresh the signature and then reselect the signature from the drop-down selector in the message composition window. (I have two signatures - one static, the other the output of my script - to get around this.)

I have used that way for creating an HTML signature in mail:

1. Create a new signature with a preferred text
2. Full close mail app
3. Go to: Library ▸ Mail ▸ V2 ▸ MailData
4. Open all files with .mailsignature extension until you find the text that you have writed in the first passage
5. Copy that file in to your desktop
6. Unlock it
7. Put your HTML code
8. Save and lock again that file
9. in Library ▸ Mail ▸ V2 ▸ MailData delete the old file .mailsignature
10. Copy your new .mailsingaturefile with your HTML code

If you don't do that passage mail will replace all .mailsingature files that is no locked or not correspond whit his signature name code.