The problem happened again, so here's what I was able to test/determine:
Once the problem happens, no email goes out to the Internet from that computer (several different servers were attempted)
DNS lookups work
From that computer I cannot ping google.com (which works typically)
From that computer, I can access the Internet using a web browser – I suspect because I have ports 80 and 443 load balanced with a different Internet connection.
I did not see anything unusual in the mail server's mail.log
I did not see anything unusual in the mail server's system.log
I hadn't mentioned before that networking internally to that server works as normal.
It feels like pfSense receives the packet for SMTP connection and doesn't know what to do with it. One thing I forgot to test was SMTP connection from another computer on the same network ( something like this: telnet aspmx.l.google.com 25 ). I'll try that next time.
Is there any way to determine how pfSense is routing a connection?
Any other suggestions?