This suggestion came up again in the Mozilla Russia forum - since emails don't have a real domain, use sender's email address at the document "domain". That would allow specific filters like "email@example.com##body", "*@example.com##body" or "@@*$document,firstname.lastname@example.org". Note that these aren't real wildcards - the only possibilities are a full email address or "*@domain", nothing else will be supported. Implementing this suggestion used to be unrealistic but now that Adblock Plus is already often checking the domain for element hiding rules itself rather than leaving this up to the browser (was necessary to implement domain exceptions) we can apply the same mechanism here.
Low priority but potentially nice to have.