[88]

Does your ISP allow you to use their service to host a server? Most expressly prohibit it in their terms and conditions.

[lucb1e]

I remember in 2005 or so, there were people that got letters from their ISP to stop hosting things, and blocking port 25 was common until about 5-10 years after that but then net neutrality became more of a topic. Haven't heard of any such trouble since then.

There are also ISPs that expressly encourage it. Formerly I was with XS4ALL who encouraged it when I asked them on the phone (long time ago), nowadays their spiritual successor is Freedom Internet. I think the name says enough :)

[Semaphor]

That is pretty uncommon in Germany (possibly illegal?), here I can host whatever I want.