There is one particular group of folks that I am unable to send mails to using mailinabox. I have spoken to their IT folks and here’s what they have said to me"
"Since our MX has a priority of 20 and MAIL has a priority of 10 (preferred), I’d suspect that somehow your system is forcing it to use MX instead of MAIL. Perhaps this was a response to problems in the past.
In theory, if you were to remove the FORCE MX setting, your system would utilize our preferred MAIL server per the SMTP email rules. I can see your domain is whitelisted on our system, and I confirmed with *** we’ve seen no email for 4x months. Just my 2 cents."