One of my company's clients is receiving non-deliverable receipts when emailing our users. We have been receiving emails from them for years, but yesterday this started happening. This is what they are getting back when they send emails to us. As far as I know they are the only clients who are having problems.
>>> Client's Name 10/14/2008 7:38 AM >>>
>>> M+ Extreme Email Engine <email@example.com> 10/14/2008 7:36 AM
>>> M+ >>>
The original message was received Tue, 14 Oct 2008 08:35:27 -0400 from firstname.lastname@example.org.
----- The following address had a permanent error -----
<email@example.com>; originally to <firstname.lastname@example.org>
The mail system encountered a delivery failure, error code -1.
This failure may be due to circumstances out of the mail systems control.
----- the delivery transcript -----
authentication required for relay (#5.7.1)
Reporting-MTA: dns; mail01.clientdomain.com
Arrival-Date: Tue, 14 Oct 2008 08:35:27 -0400
Can someone tell me if this problem is on my side or my client's side and what the fix for it is?