I've got a recurring problem with a windows XP box running outlook 2003 SP2 - occasionally (sometimes once a day) the XP box will have problems connecting to the local mailserver (a linux box running postfix). Outlook throws up a message saying that the system timed out attempting to connect to the SMTP server. The guy using the box seemed to think that it seemed to happen when he replied to messages (not every time, obviously), but I can't see how that can be anything other than a coincedence. When this particular error occurs, no matter how many times you try outlook fails to connect to the mail server. If you close outlook down and open it back up, it will send your messages, but it won't download any other new messages until you restart the computer.
I saw some suggestions on the web that said 1) update your software (it's already up to date, though) and 2) increase your timeout settings. I turned the timeout up from 1 minute to 2.5 minutes, but it still happens. I'm confident it's not a network latency issue - there is no way that it would take 3 minutes to communicate with the server. Nothing else is experiencing a timeout problem or even a slowdown.
We don't have any other computers in the office running outlook 2003, so I can't say if it's an issue isolated to that computer, or an outlook issue. No one else has any problems connecting to the mail server. Everyone else in the office uses Eudora, Mozilla or Thunderbird mail, with the exception of one person running WinNT with Outlook 2002 SP2.