They must. Verizon now blocks port 25 and does not allow you to opt out. Why do I care? Well our hospital just opened up free WiFi everywhere and when I found I couldn’t send email I made a couple of trips to the Information Services folks.
They swore up and down that they weren’t blocking anything at the router. An epiphany later and we figured out it was the Verizon DSL they were using for the access points.
Well, the simple solution is to change email to accept SMTP on port 587. This was simple enough to fix on the server. Just uncomment the following line in /etc/postfix/master.cf
#submission inet n - n - - smtpd
Then sudo postfix reload and you’re good to go.
Of course I also had to change my iPhone SMTP to use port 587. The only tricky part was remembering that I have my router set to provide a basic hardware firewall, in addition to the server’s ipfw. I had to open a service and a rule on the router to let TCP traffic on port 587 through.