Not a very common problem. But if you experience this issue please try the following:

1. Disable any antivirus software or disabled outoging scanning of emails. This is one of the most common causes of this symptom.
2. The emails may be corrupted. Try moving the email messages to the drafts folder and send a new message.
3. Click Send/Recieve and see what occurs.
4. Try opening and closing the Email Client may also help.
5. Ensure that the mail program is NOT in offline mode.