When my users send email with a PDF attached to one domain they get an NDR (all other domains we send email with PDF attachments receive them fine). However email without PDF attachments goes through fine. I thought they must be blocking it on their end but they tell us that they are only having this problem with us. Any ideas on how to troubleshoot this?
Microsoft Exchange has been trying to deliver this message without success and has stopped trying. Please try sending this message again, or provide the following diagnostic text to your system administrator.
4.4.7 QUEUE.Expired; message expired ##
Thanks in advance for any advice :)