I was compelled to especially after reading all of the useless banter that people were responding to (as per usual) but meant no offense just a simple quick answer. I figured that someone watching the thread might be able to give me a pointer...
I don't know what you're talking about 'top post' I replied to one of the other posts, nothing more, nothing less. Again not trying to offend anyone and get more useless crap on a list I would hope to be focused on technical things...
See http://www.river.com/users/share/etiquette/ for a description of top-posting and and what's wrong with it. By not top-posting and properly trimming quotes, people will be able to understand your post in the context of the thread more easily and you're more likely to get a useful response.
I'm using a web based application OpenGroupware (on ssl) with postfix and cyrus-imapd on the backend; however I could use either Outlook or Evolution to connect to. I wasn't sure if it was a server based method or on a client (such as you mentioned). However if a server based signing is possible I would prefer that.
Evolution does GPG signing and encryption very nicely, and should be comfortable to use for anyone familiar with Outlook.
Paul.