Delivery to the following recipients failed

[npinfotek]

I have a user who tries to mail someone (outside our organization) and receives this message:

>>>This is an automatically generated Delivery Status Notification.
>>>Delivery to the following recipients failed.

I also know that the person outside our organization gets this when they try to send mail to us:

>>>>Unable to deliver the message due to a recipient problem
>>>> MSEXCH:MSExchangeIS:[SERVER_DOMAIN]:[SERVER_NAME]

All other mail flow is OK (mail gets to us, and our mail reaches its various destinations).

Does anyone know how to repair this?

 

[remain]

what are the number errors that it gives you?

 

[npinfotek]

There were no numbers - only those statements.