One of our users has been sending emails to a particular client with no problem for the last year. Suddenly, today, he started receiving Undeliverable - 554 Transaction Failed messages for some of the emails; however, some are still getting through. For example, he sent one test message to two users at the client office; I checked our outgoing Barracuda box and one of the messages has a status of Delivered; the other has a status of Rejected.
Any idea what could be causing this? Is it likely on their end or ours? It seems the messages are going out fine from our end, but should I be checking something else?
Thanks in advance.