Several people in the Apple Discussion forums have reported 
that they are unable to connect to their mail servers after applying the 10.4.7 update.
The problem is (usually) that Mail.app’s new ability to connect through a SOCKS proxy had been turned on during the update.
The fix is simple.
Open up your the Network pane of your System Preferences. Choose your current method of connecting to the Internet (Built-in Ethernet, Airport, etc) and select the Proxies tab on the configuration dialog.
Make sure that the SOCKS proxy box is unchecked:
I’ve just upgraded and not had this problem.
Apple has already taken my SOCKS off…
actually, something random: my son has his gmail running through mail.app, and during the upgrade it appears to have erased the password for all outgoing mail (checking was fine, but sending prompted password entry)…not sure if this is happening to entourage users, or others not dealing with google’s servers…
Since the upgrade, my IMAP account is the only one acting up. It will check mail on the first occasion after the app is launched, but then refuses to, even if I manually ask it to check mail. SOCKS proxy is off. Strange.
Brilliant – Thanks a million!
My gmail also didn’t work, removed the socks settings, and there it works again!!!
Thank you! I finally got it working again.
Apple should have made this funtionality an option in Mail.app preferences.