[snowwrestler]

This is not a workable solution because the problem is not based on what port SMTP is running on. Organizations and ISPs voluntarily subscribe to email blacklists because they are desperate for any help in reducing spam volumes. They would filter email coming from blacklisted IPs regardless of what port it came in on.

You're basically proposing a whitelist solution, which has many known problems: it does not scale well; it does not handle new or unexpected email partners; it relies on the simultaneous cooperation of all parties; etc. In this particular case it also relies on spammers remaining ignorant of the new port for SMTP--which seems unlikely.

[overbroad]

The problem is the open internet. There are ways to establish connections to a peer-to-peer overlay that take this out of the equation. The ISP is unlikely to block UDP traffic on some high port. Throttle perhaps, but not block. That gets us around the port issue. Once you log on to the overlay, the ISP only sees one port.

Then we are free to do our SMTP of other messaging as we desire. Each connected machine can choose what ports it wants to listen on, if any.

And what if this does not need to scale? What if it's only being used for a small group of people? What if all the people know each other? A very specific but very common use case. Not everyone is a celebrity with a gazillion "friends". Nor is everyone constantly conversing with new acquaintances. Some people have old friends and family. So I've heard.

Is it worth the spammer's time to try to find an SMTP daemon for each indivdual email address? Under the current system, things are centralized enough that a spammer can spam hundreds, thousands or even hundreds of thousands of recipients via sending to a single SMTP daemon.

Spammers have to send enormous amounts of spam to be successful. Having to do extra work to find an SMTP daemon just to send email to one recipient, and have to do this repeatedly, seems like it would not be worth a spammer's time. At least, not when it's so easy to just spam people that are using email the usual way: allowing some third party to handle their mail.