If your having problems sending mail to a particular store user but OWA still works.
You receive an immediate NDR with the following text:
There’s a problem with the recipient’s mailbox. Please try resending this message. If the problem continues, please contact your helpdesk.
You may have a store corruption issue.
- To resolve either move users to a new mail store
- run isinteg –fix –test alltests on the database (this will require down time)
Another possibility (same solution is that there are too many named properties (look for 9667 events) – in this case you have to move users to another store\database.