I have 2 users getting the following message from the same external email server for 6 recipients:
This is a delivery status notification, automatically generated by MTA domain1 on Thu, 24 Jan 2013 14:16:18 -0500 Regarding recipient(s) : firstname.lastname@example.org Delivery status : Failed. None of the mail servers for the destination domain have so far responded. (domain1.com).
We have been attempting delivery for 5 times.
Maximum delivery tries attempted. Please contact your administrator to contact the destination domain or resend your message. Delivery failed.
Sender : email@example.com
Recipient : firstname.lastname@example.org
MTA Response :510 The original message headers are included as attachment.