A great deal of Cyrillic spam has made it into my various inboxes lately. I don't read Russian, so I'd sure like to be able to configure my email accounts to not accept stuff I cannot read, but this isn't an option‽
Configuring acceptable languages has been an underused but nonetheless available feature of HTTP for at least a dozen years. Why can't MTAs be configured to look for character encodings that the user cannot use? This would be a simple, straightforward solution to most of the spam that somehow currently makes it through my filters.