Yes they do. And thus botnets are forced to route via the ISPs intermediary server instead of making a direct connection to the recipient mail server. That intermediary server can apply rate limiting and blocking.

ISPs who implement blocks on port 25 and force everyone to use their mail server are able to take responsibility for the spam leaving their network.

Note, ISPs don't block ports 587 and 465, so they're not preventing you from using services like GMail etc.

Also, it is completely relevant as they have all done exactly what is being proposed in the article being discussed. ISPs in South Korea will also provide mail relays for their subscribers to use.

But this is really easy to work around. Isn't the Soth Korea arguing that all mail relays should block port 25, not just those on residential networks?

I haven't seen them say that anywhere. If they block port 25 everywhere, then they simply prevent the ability to send or receive email within South Korea. That's clearly not their plan.

The port 25 block is designed to force compromised systems to route email via controlled boxes. The fact that you can route email via controlled boxes doesn't mean you've worked around the block, it means you've been forced to do exactly what the intention was.

Maybe I've misunderstood something. I wonder how much spam is sent via compromised residential systems and how much is sent via less than perfect commercial systems?

The vast majority is from botnets, which compromise mostly of compromised residential systems. http://www.theblaze.com/stories/email-spam-down-82-percent-s...

Wow, just tested, my home ISP doesn't block outgoing TCP port 25. I thought it did.

I suppose that would be a form of non-net-neutral corporate censorship. But one could make a pretty good argument that it's a technical issue with a core protocol of the Internet. It's not really practical to send emails from your home anyway because so many servers will simply refuse to accept them from residential netblocks.