HenBen - why is it impractical to say "Use strong crypto"? It's a single checkbox in any of the common email clients; web browsers have had it for years and paranoia at large corporations has at least been enough to get most other remote access protocols to either use SSL (e.g. Microsoft Terminal Services) or play nicely over a VPN. 5 years ago that was a reasonable complaint but now it's rare to find a situation where the use of an insecure protocol isn't due to laziness or incompetence.

As far as spam goes it's hardly a big deal to limit normal outbound SMTP to a single authenticated server - most ISPs are starting to do that anyway to cut down on open relays and things like those spam-sending worms - and it's increasingly likely that the average DSL / cable line is going to be on the non-MX blacklists anyway. It's not at all unreasonable to say that you must use SMTPS or a VPN to send email from a public access point if you don't have an account.