Seems they can't verify your "From" address

HCL Notes/Domino 8.5 Forum (includes Notes Traveler)

Subject:

I suspect, other side tries to check your From address for validity but fails. There are 2 main checks - check if domain part of your From address exists in DNS and check for your From address validity via SMTP. So most common reasons are: DNS problem (i.e. they can't resolve your domain name entirely), malformed From address (check it at your side), antispam/firewall filters before your Domino, that prevent other side from checking your From address via SMTP (so some kind of deadlock occurs).

Feedback response number WEBB8NHEAS created by ~Lorraine Pregerovitchakoi on 11/11/2011