If you are a residential user of broadband internet access, your ISP probably has terms of service that include prohibited uses like this (taken from Comcast's TOS):
The TOS almost certainly includes language restricting you from acting like an ISP (again from Comcast):
In other words, you can't sell or even give away wi-fi access to your neighbors.
But we all know people do this. It's seldom with commercial intent; usually just a few friends and neighbors sharing a wi-fi connection. As for mail servers; well, pretty rare, but now and then a geekish type will put up a mail or ftp server behind a home connection. If the ISP blocks certain incoming ports, SSH forwarding or tunneling can bypass that. Are they going to "get in trouble"?
I think that's pretty unlikely. After all, what the ISP really cares about isn't that you are using a specific incoming port. Their concern is that you will take too much bandwidth. If you are sharing wi-fi with your neighbor, they might object to the lost business, too. But unless there is some other reason for them to to notice you and object to your use of their service, minor use of prohibited ports or letting someone else piggyback your wi-fi is likely unimportant.
On the outgoing side, some ISP's now block outgoing port 25 unless it is to their mail server. That's a anti-spam measure to prevent your machine from being a pest, but it is annoying for someone like me who has to test smtp manually now and then. Fortunately my current ISP doesn't do that to me.
Are you "violating" your ISP's TOS? Do you think you are likely to "get caught"? Does it matter?
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