Agree with NormanS
it appears the 18.104.22.168 IP is blacklisted by Sorbs, but not by any other blacklist service.
Just to double check go to »www.whatismyip.com/
to verify you IP address.
Since the reject message asks to reformat the message have you tried that? Leave the numbers off the subject line and do a simple test message and see what happens.
SNMP error 554 is kind of a generic "we don't like it."