Just to add my 2 cents here. I have recently set up my own mail server and finally installed squirrelmail last night. COuldn't have been easier.
The only problem I ran into was that I was able to receive mail and send mail to users on my own server, but couldn't send out as I would get relay access denied. After looking through the mail logs, I noticed that it listed as attempting to connect from something along the lines of res.rr.com (the ISP). I added this to the "my networks" configuration in postfix and it solved that problem. Off hand, I am not sure if this was the correct (or safe) way to 'solve' the problem, and perhaps someone else can comment on this part.
But, I would give my recommendation to squirrelmail too!