I'm having an issue with one of my Exchange Online customers. When users from a specific outside domain send mail to them they get a Delivery is delayed message back from their mail server. The mail will eventually get through to my customers but it can take over 24 hours.
I have found that when this outside domain sends mail to domain.microsoftonline.com the mail gets delivered immediately. This must mean the issue is something with DNS.
DNS settings look correct when troubleshooting the problem. When I use mxtoolbox.com it shows mail.global.frontbridge.com as the MX with a pref of 0.
The outside domain email admins have said the problem is not on their end. Strange that it only causes a problem with one specific sending domain, but they assure me it's not their fault. Microsoft has said the issue is not on their end. I have a ticket open with the DNS host (5qcommunications.com) to see if they are experiencing any problems. Have not heard back yet.
Any help or insight is appreciated, if more information is needed please let me know.
Thanks!!