Wow, I didn't think that would matter, so discourse is pulling the hostname instead of making the app.yml setting email the priority...I think we found a flaw in discourse here boys.
I've already made an exception in main.cf of those settings (and permit_mynetworks):
mynetworks 127.0.0.0/8 172.17.0.0/16 192.168.2.0/24
I'll see if I can resolve the username@address problem.