Example, I send an email from email@example.com to firstname.lastname@example.org and receive the following undeliverable message.
Delivery has failed to these recipients or groups:
A problem occurred and this message couldn't be delivered. Check to be sure the e-mail address is correct. If the problem continues, please contact your helpdesk.
The bouncebacks were coming from our Exchange server. You can see from the message that it fails because it has somehow appended my sending domain name onto the end of the receipients emails address!
Each time this had happened, the user had populated the To: field by either clicking on a mailto: hyperlink or copying the email address from elsewhere in the email body and pasting.
It's a known issue. After installing Office 2010 SP1, you will then need to apply the following hotix http://support.microsoft.com/kb/2597052. Please note that you will need to reboot the machine once installed, altough you can select to 'Reboot later'.
In Outlook, you will also need to delete the auto complete suggestion of the affected recipient. Start typing their address into the To: field, once the suggestion appears, click on the X to delete it from the cache.