This practically always is a wrong setting. But it also could be some problem with your antivirus if you instructed it to check outgoing mail. And in certain exceptional cases it's a problem with your Sent Items folder.
A good start to a solution would be to tell us the exact error message. Unfortunately, Cnet doesn't require its mods and members to be clairvoyant.
I have set up outlook express with my email provider (using their settings), i can receive, but will not send. Keep getting an error message. any suggestions? could it be a preference within outlook express?