I realised after I posted the last one that I hadn't really stressed the fact that I felt it was important to establish whether your Zen Cart shop could now talk to the local mail server. That would be a very good step 1, as if not, ther's some more config to do on your Ubuntu setup.
i.e. Set mail transport to PHP, send an email from admin, and then check the mail log to see whether it is accepted by Postfix.
Once that is accomplished, we know you have a working shop, and several ways to get the mail out to the world.
And yes, in this day and age mail servers can get very picky about the connections they do and do not receive and accept. Thats why I am so keen to get the mail as far as the Postfix server.



