Outlook 2003 and exchange server…

We’ve recently migrated our Network. This included Migrating our mail server from a Linux system to exchange 2003.
Initially everything went smoothly and the mail came in and went out quite nicely.
Now there are 1 or 2 users complaining that they can not send mail. It’ll just remain in their Outbox.

They’re still receiving mail fine and when you do a “send and receive”, there are no errors on sending… yet the mail remains.

Can anyone please shed some light on this issue for me?

Thanks guys and gals.