South Korea is reportedly preparing to block TCP port 25 and force all mail traffic through "official mail servers", whatever that means. The BBC article isn't much on technical details. I'm still on the fence about blocking port 25. Fundamentally, I'm opposed to blocking ports outright. If somebody is shown to abuse a service, I'm fine with restrictions. I'm not excited about the prospect of requiring all mail to go through a central server, whether it's mine or someone else's. Consolidation like that tends to make small problems into big ones, such as when one user does slip through and send out spam, now everybody else on that server is put on a blacklist. I can tell you, that's no fun to clean up.
I'm also not convinced that blocking port 25 eliminates spam. It closes down one loop hole, yes, but spammers just need to find a single vulnerability. The common one I see now is to steal mail passwords and use the victim's legitimate mail servers as a relay. Using a good password and not sharing it between sites is good protection against that, but not everyone has gotten that memo. Destination end filtering, as imperfect as it is, will be around to stay for a long time.