Originally Posted by
MarkD
Mxtoolbox (a very handy resource indeed) does not show a DNS record for plus.com (assuming that domain is where you are sending)
this is not your issue - it’s your recipient’s issue. Give them a call - I doubt they are getting email from anyone
You got that wrong I'm afraid.
This is the recipient server doing a reverse DNS on the sender IP 40.92.55.37 (a very common check) and not finding a PTR record.
The sender being a Microsoft SMTP systems that is possibly misconfigured in their DNS. The only way to fix this is either for Microsoft to make an entry for said IP or for the user to retry in the hope that a different, hopefully correctly configured server will be used. An alternate explanation is there might be an issue with the resolver used by plus.com as the IP currently resolves to a Microsoft server in Oslo, Norway. So either they fixed it since you posted or plus.com doesn't know how to do reverse DNS...