Mail won't send messages

My partner has a G4 with OS X 10.3.9.
Recently lots of messages have refused to send from her Mail Outbox,resulting in the App beachballing/having to be ForceQuit. Sometimes she gets the following error message:-
'This message could not be delivered and will remain in your Outbox until it can be delivered.
Verify that you have addressed the msg correctly.Check smtp settings in Mail preferences and verify any advanced setup with your system administrator'.

I've tried the following things:-
Repair permissions
Zap PRAM
Rebuild Mailboxes
Tried trashing Mail prefs but when she tries to log in again with the correct details,she gets an 'unable to connect' msg,so we have to drag the old prefs back out of the Trash

Any ideas?

sounds like your "outgoing mail" setting are off. check with your IT guy to make sure this hasnt changed recently. we run into this all the time where i work.