From just one user mailbox so far, whenever an email is sent to either internal or external recipients, I receive a NDR -
Undeliverable: Delivery has failed to these recipients or distro lists:
Generalting server: exch.domain.local
#550 5.1.1 RESOLVER.ADR.RecipNotFound; no found ##
1. The emails are delivered successfully to both internal or external recipients
2. The email firstname.lastname@example.org wasn't included as a recipient. It doesn't exist, and from my historical records, has never been a mailbox in domain in the last 3 months. This only started happening today.
There is no email forwarding on this mailbox.
The only thing I can think is it is related to the postmaster logging, but then you would think this would happen for other mailboxes.
Any ideas ?