Since upgrading to Mac OS X Sierra, the following AppleScript does not work for adding a signature to an e-mail in Apple Mail.
I haven't yet found a workaround, so thought I might quiz the group.
No longer working:
set message signature of msg to signature theSignatureName
I submitted a bug report along with lots of info that Apple requested. After about a week or so, here's the response I received.
Engineering has determined that your bug report (28851572) is a duplicate of another issue (28497651) and will be closed.
I'll continue to track this as much as possible, but for now... the ball is in Apple's court.