Is there a way to setup authenicated access from the internet to my GWIA

The idea I have in mind is to NAT port 25 and assign it an DNS name to the
internet however I want only want user that can authenicate to the agent to
have access as I already have SMTP open though my regular mail exchanger DNS

The problem that I'm having is that I already do this though my Barracuda
which utilizes SMTP auth proxy however the situation is that it works for
everyone expect iPhones, for some reason it works at time and other times it
doesn't. As it's configured now, my Barracuda accepts email if proper
creditials are provded (Gw user name and password) however it (the
barracuda) doesn't log that info SMTP auth info, for diagnosing the problem,
I'm trying to go directly to the source however of course I don't want to
bypass my Barracuda email firewall and have a wide open SMTP host on the