When we send email to external emails addresses or if people send to us it works correctly. However if a user in the system send s to another user in the system using the firstname.lastname@example.org it fails with the following error. Why? The user does exist. The email address email@example.com is associated with the user account.
The recipient's e-mail address was not found in the recipient's e-mail system. Microsoft Exchange will not try to redeliver this message for you. Please check the e-mail address and try resending this message, or provide the following diagnostic text to your system administrator.