I was with same issue and solve by myself
I change the postfix port to 587 because here in Brazil the 25 port is blocked (to block spammers)
nano /etc/postfix/master.cf
I edited:
##smtp inet n - n - - smtpd
587 inet n - n - - smtpd
So I could to send emails but not to receive.
The trick is Firewall > Port forwarding > redirect port 25 to 587