It's not even that complicated. I have SBC DSL and run my own Sendmail server for my own use. When SBC started blocking 25, I simply had to fill out a web form to get it unblocked. Today I still run my own mailserver on 25 as if the blocking didn't even exist...no smarthost required.
If the ISP makes unblocking the port that easy then I'd have no problem with it. My worst nightmare would be having to call the "Your call is important to us" recording to get in re-opened.