> The domain name is OPMAN.COM if that helps.

Also check for any MX hosts and that each of the MX host(s) does not
terminate the SMTP connection when the SMTP EHLO command is issued.

BTW I have experienced routers that do not properly frag and defrag large
(>1500 byte) datagrams, which might cause list messages to bounce, but
direct e-mail to be OK, since different routers could be involved.