I've recently installed a new server with Domino 9.0.1, and configured everything as I always do.
Everything was working fine until today, when with just a single domain we're receiving this error
Error transferring to THATDOMAIN.COM; SMTP Protocol Returned a Permanent Error 550 without authentication
I tried to google it, search on forums but I couldn't find anything that matches my problem.
Two days ago we have configured a Reverse DNS since we had problems with that too, but now it's working fine. Other domains are routing fine.