A client of mine sent out a mass email to a bunch of people on his contact list. A bounceback email was received by my client stating that: "The server has tried to deliver this message, without success, and has stopped trying. Please try sending this message again. If the problem continues, contact your helpdesk."
That error is associated with every recipient in that mass email. Any ideas on where to look to resolve this? He is able to send and recieve other emails normally.