As it turns out - there were many issues that I chipped away at and it boiled down to this:
Using this article as a guide
- which is excellent, but leaves a few things out. I managed to get everything up and running - but couldn't connect via port 465. What the guide doesn't tell you (what I only found through searching for the answer) is that you must go and un-comment the following in your /etc/postfix/master.cf. I guess the guide's author assumes you know to do it..
smtps inet n - - - - smtpd
I had un-commented the second two lines in the beginning - but not the first.
This in combination with some port forwarding to 465 in the router was the solution to getting all of the other mail clients to connect to the outgoing SMTP server. I still have yet to test the Apple Mail.