I have recently came accross the problem. One of my customers had wrong setting of no reply e-mail address and messages got lost between on-premise hub transport servers and Symantec cloud in the way, that it appeared message has been sent successfully, but dropped on Symantec side without notification.
All noreply email addresses has been set in MailContact object as follows: smtp:firstname.lastname@example.org,smtp:email@example.com …. SMTP:firstname.lastname@example.org and ExternalEmailAddress: email@example.com.
It means that all messages outside the company send from firstname.lastname@example.org,smtp:email@example.com aliases has been sent with ReturnPath firstname.lastname@example.org and due to same reason (non existing domain and recipient) messages were dropped.
- Change External email address of Mail Contact Object to recipient from existing domain
- Create transport rule to drop messages to this recipient
From my point of view very ellegant and easy.
- Remove Mail Contact object
- Create DL, enable non authenticated senders to send messages to this DL
- Assign noreply addresses to this DL
- Use external e-mail address as primary SMTP address
- Do not add any members to this DL. This step will cause to drop all incoming noreply messages without notifying anyone.