I can post the outcome of /var/log/maillog It doesn't show much, except for the main domain that is working correctly...the few emails I have sent through the server on that domain.
I would like to say yes to the second question, as I used the ISPConfig wizard to create this, where an MX record was auto-generated. But just for fun, since its' auto-configuration didn't work, I changed it around here and there, but nothing works. Presumably it would need to point to the only server on the system? The originally working domain has MX records of hostwithsupport.com pointing to mail.hostwithsupport.com, and the new domain's ISPConfig auto generated MX record was domain.com pointing to mail.domain.com.
Funny thing is that ISPConfig also auto-generated the A record according to the correct info that I input for the new domain. I can't even get the site live. Trying to start bind showed that the A records were invalid for the new domain, yet they are identical to the original domain which is live.
/etc/init.d/named restart (or start)