It's a 'lawful application or service' and not all ISPs block Port 25, only some. For instance, my cable ISP (Time Warner) does not in this market.
I do question whether blocks on inbound ports at the ISP level would also be considered permitted for the same reason. Running a web server on your residential connection isn't unlawful, but plenty of ISPs block it.
I won't be the only one to ask these questions I figure.