fo?
1. If you are using Firewalls or Norton internet security etc, please
disable them and try sending emails. If you are using any antivirus that is
configured to scan incoming and outgoing email, please disable that
integration with Outlook or you may even have to uninstall/reinstall
(omitting the AV integration with Outlook).
2. Follow these instructions:
a. At Tools menu, choose Account.
b. Choose Mail.
c. Click double to your Account.
d. Choose Servers.
e. Click My Server Requires Authentication.
f. Choose OK to complete.
Then try sending email.
3. Check this document for more reasons and solutions:
http://support.microsoft.com/?kbid=813514