I have been looking at other threads on this topic, without success.
I get the usual rejection message:
Remote host said: 554 resimta-ch2-03v.sys.comcast.net resimta-ch2-03v.sys.comcast.net 2002:d1a4:11a::d1a4:11a Comcast requires that all mail servers must have a PTR record with a valid Reverse DNS entry.
1) I have a valid PTR record and valid DNS (checked and double checked!)
2) My servers are not configured for IPv6 (checked)
3) This issue is unique to Comcast email recipients