Can a Heroku app use different/multiple ports?
No, just running some quick tests against the heroku proxy (proxy.heroku.com) I can only make connections to ports 80 and 443. Being as heroku don't provide any POP/SMTP it doesn't make sense for them to have those ports open I'm afraid.
Web services on Heroku normally use the PORT environment variable, but I want to run a custom mail server on Heroku. So I need to expose 2 ports, preferably 110 and 25, but I can live with alternatives. Is it possible?