Here is what ISPConfig created as the domains DNS MX record:
MX 10 hostname.domain.com.
After I changed it to this it started receiving email:
mail.domain.com IN MX 10 mail.domain.com.
The reason the other domain were working is because I imported the dns records from an older version of ISPConfig from an older server. I think there is an issue with this version of ISPConfig and how it writes DNS Records - or an ISPConfig Setting I need to change so I dont have to manually enter all my records...
Thanks, and cheers!