We have a very important client who when they attempt to mail a user on our domain irregularly get's bounce backs with the message:
#550 4.4.7 QUEUE.Expired; message expired ##
The client in question does not appear to have issues mailing anyone else on our domain or any other. I receive all E-mail they send to me. The user here also has no other issues with his mail. As I said, the issue occurs irregularly: some days it's fine, others it's not.
Does anyone know if this is an issue our end or theirs please?