We're experiencing a strange problem in that a client is trying to send an email to one of our users and they are getting an NDR with the 552 error (Storage Allocation Exceeded error). The user receives emails from other domains/senders just not this 1 particular client. I've checked the mailbox and it's not close to being full as this user is pretty good about archiving and cleaning up their mailbox. Any thoughts, ideas, and suggestions are welcomed.