I'm having very similar problems. I just purchased a 13.3" unibody MacBook Pro.
The email accounts I have setup are 2 addresses from a personal domain through Google Apps, 1 mobile.me, and 1 Exchange IMAP. After setting up all the accounts earlier today, they worked like a charm at my office. I bring the computer home, and now when I send emails from any of the accounts, the Sent spinning wheel turns, and either the message eventually sends after a long period of time (2-5 minutes) or it times out.
This is very, very frustrating! Why would it work earlier today, and just stop all of a sudden? When I test the connections, all of the Incoming Servers connect right away, but all of the Outgoing Servers do the obvious, they sit there attempting to connect for 2 to 5 minutes and either do or eventually time out.