User B has full mailbox rights to User A's mailbox.

All working fine for months.

All of a sudden user B gets "you do not have permission to send to this recipient" error for every recipient they send to.

Checks rights to mailbox of User A.

User B still has full mailbox rights, however, does not appear in the "Send on behalf" delivery options screen.

Go to add User B - "an object named user b cannot be found"

Yet user B is enabled and working fine on the domain.

I can add User B to a security group - it finds them then, just not in the "send on behalf of" bit. There are no limits to their mailbox either.

I don't get it Has anyone seen this where the user exists in the AD but you can't locate them