Without knowing any details it's hard to say. Please advise the following:
Type of mail account
Is AV configured to scan incoming/outgoing email? (If it is, disable it and
try sending again. This won't compromise your security since your real time
scanner will catch anything it's been updated for)
How often are you polling for mail? (If it's more frequently than 10
minutes, change it and try again)
Have you checked the Application Event Viewer for any Outlook or
Office-related errors?
Are there any troubleshooting steps you've taken so that no one advises you
to do the same thing over again?
