Many users seem to be getting errors sending emails using Microsoft Outlook 2007
Emails seem to be received successfully.
To resolve the problem try one or all of the following :-
1. See if you can send the email using Outlook Express
2. Try removing any website domain names (URL’s) from your signature
3. Try setting the MTU in your router to 1365
4. Make sure port 25 is open in the router settings
These seem to be the cause of this error, but it only seems to happen in Microsoft Outlook 2007