I'm having problems with a particular user trying to send to an external contact. The external contact has sent an email to my user but when the user tries to reply or send a new mail to this external email address, he receives a mail delivery failure. I can email the external address with no problems. He can email other accounts in that mail domain - it is just this one he has problems with. Anyone any idea why??
Error as follows:
The message could not be delivered because the recipient's destination email system is unknown or invalid. Please check the address and try again, or contact your system administrator to verify connectivity to the email system of the recipient.