I have two Exchange 2010 servers running on Windows 2008 Ent R2.
These mail servers have been running fine for a few years.
Today I noticed two things.
1. users were telling me they were having delays receiving emails from outside of our own domain. The mail gets sent out, but it takes about 15-30 mins for users outside our our domain to get their mail. Mail sent from inside our domain gets delivered right away.
2. An error message that I see when I go to Tools->Queue Viewer in EMC. The error is: 451 4.4.0 DNS query failed. The error was: SMTPSEND.DNS.NonExistentDomain. This error shows up when you click on the "Queues" tab and then look at the "hub version 14" under "next hop domain" column.
I'm assuming these two things are related. I don't understand why the problem is just showing up now. As I said, mail delivery has been fine for a while and I haven't done any major updates to the server in a few months.
Thanks for any ideas and suggestions as to what might be causing it and where I can look.