Problem here is that people can't use their ISP's mail server without also using their ISP's email address. Add port 25 blocking and server blacklisting and we solve spam by blocking all email :D

Something similar happened a short time ago here in Tokyo - an ISP said that they were blocking outgoing port 25 and would you please use our mailserver. No problem - only one setting to change. Then, mail got bounced with "Not a biglobe.ne.jp address". I'm so happy I don't work at their support desk.

But you can use a remote server on alternate ports. For example, you can use your Gmail account for SMTP on secure ports, which are not blocked. There's no reason that others can't do the same (many do).

They certainly can, but it's a major headache for the ISP's support desk. My mail provider offers an alternate port but it's request only info - they don't publish it probably on the grounds of those who know enough to ask also know enough to use.

No, the ISP wouldn't do that, the mail provider does it. For the 99% who uses their ISP's mail servers, this won't be an issue. Anyone who wants to use a foreign mail server/account would then have to learn how to send mail through THAT account. Google gives really easy instructions on their site, so anyone else could too.

My ISP, in France (Free) recently blocked port 25.
You can still open it by a console if you want it.

I think that's a good measure, because it isn't very intrusive, and will block a lot of zombie machines, whose owners had no idea of what was happening.