Yes I've already created the mail users and they are in my /etc/passwd.
Is there anything in any configuration file that may point to why I can't connect?
E.g. I've noticed that there are two entried in my /etc/postfix/main.cf for the same key.
mydestination = ispconfig.posted-jobs.com, localhost.posted-jobs.com, , localhost
AND at the very bottom of the main.cf there is another
mydestination = /etc/postfix/local-host-names
I didn't add that second one.