I do not know if this has been covered yet?
I was struggling to get the outlook plug-in to work with MS Exchange, the issue is exchange re-writes you mail account to user@local instead of user@yourdoamin.
SO what I did was, setup a dummy pop3 account,I set it to use a invalid pop3 server say 22.214.171.124 or 100.100.100.100 or what ever.
Then I use my exchange server as the SMTP server.
Then I go to the send receive options in outlook and tell outlook not to receive mail for this account, only to send.
So now I have my exchange account and my “fake” pop3 account, now I make the pop 3 account the default account and when I click send it will use the correct email for the PGP key I.e. user@yourdomain and not user@local.
If any one is interested, I can make this a howto doc with pic’s etc.