Problem: When my users are trying to send an email to Comcast, aol, q.com, etc.. they would get an error that said:
“Unable to deliver message to: <Person@comcast.net> Delivery failed for the following reason:
Unable to read SMTP greeting from mx2.comcast.net[184.108.40.206]
This has been a permanent failure. No further delivery attempts will be made.”
I checked RBL’s and we weren’t on any and Comcast didn’t have us on their black list. This wasn’t happening with all email domains but with enough to get complaints.
Solution: Found out that my ISP changed the way it handled DNS so revers lookup wasn’t working. Many institutions won’t accept email if the sender doesn’t have an entry for reverse DNS. I contacted my ISP and they fixed the problem.