A user forwarded a message to the Help Desk with the following contents:
Delivery has failed to these recipients or distribution lists:
Recipient's NameYour message wasn't delivered because of security policies. Microsoft Exchange will not try to redeliver this message for you. Please provide the following diagnostic text to your system administrator.
The complaint was that anybody who emailed the recipient always had their email returned back to them... except the Help Desk.
The problem and solution lay with the distribution list in Exchange, and yes only the Help Desk was authorized to send messages to the Recipient.
If you're facing this problem, the distribution list in Exchange can be checked by right-clicking on the Recipient then click on properties. Select the Mail Flow Settings tab then double-click on Message Delivery Restrictions. Simply add users or tick on All Senders.