I have three Macs checking the same email accounts, and as such, love the IMAP system -- never read a message twice, flags stay flagged, junk stay deleted, etc. However, I have been using a dead POP3 account just to send emails from my personal domain. I was getting fed up with this, so I tried to use commas to make more "send from" addresses for the .Mac account in Mail's preferences. For some reason, it only worked for a little while (like minutes). Note that emails have to be sent from a non-.Mac SMTP address -- here in the UK, my mail is sent through my broadband provider just fine, and retains my personal domain address.
So here's the hint. I simply set the IMAP account manually in the new account pane, using the mail.mac.com for incoming mail with the usual username and password. Now I can choose what address the email is sent from. This has worked great for a few days now, and so I thought I would share. So far, I have found no side effects or problems; if you know or think of any, let me know.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2006110705365729