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."
Makes sense. At the very least they (the target domain) shouldn’t have an MX record of whatever priority that cannot receive mail. Glad you were able to clear this up for the original poster.