Interesting problem. I installed the beta last week. I started to notice that not all my SMTP mail is coming in.
Some message get through but not all and they queue up on my upstream host.
I have a DNAT rule for the built in SMTP definition to my internal mail server.
My SMTP proxy is off and I could not connect from an external host on port 25 at all. Have made a new definition for Port 26 and set my upstream host to route to port 26. Mail is flowing now no problem.