We are presently running into problems with a single user attempting to email shared mailboxes.
The shared mailboxes are configured to allow anyone to email them, and do not require that all senders are authenticated.
We have attempted to recreate the user’s mailbox and move it to a separate database with the same results.
When this particular user emails the mailbox they receive an NDR which reads as follows:
Delivery has failed to these recipients or distribution lists:
There's a problem with the recipient's mailbox. Microsoft Exchange will not try to redeliver this message for you. Please try resending this message, or provide the following diagnostic text to your system administrator.