Comment by 9ac345a5509a

One such system is Hashcash [0], there is an extension for Thunderbird [1] which uses it.

The only thing is that legitimate mass emails (think mailing lists) will be delayed as well, unless some sort of exception is added.

  [0] http://en.wikipedia.org/wiki/Hashcash

  [1] http://pennypost.sourceforge.net/PennyPost