alt Hacker NewsRandom unpolished idea: a (local) search engine that runs when seeing the email and stores the keywords encrypted in its index..
So if you're looking for "Nigerian prince" it will look up "Avtrevna cevapr" and return references to the emails containing that term.
Is that an April fools joke? Proper encryption suites don't produce something that looks like a Caesar cipher, it's just a solid block of seemingly random data. You can't really index something like the words inside an email unless you first decrypt it.