Anything tbh.
It particularly tickles my fancy when someone praises my kinks, or favorites.
On an unrelated note, the hottest kind of mail (I) could get is probably A) someone wanting to dirty talk, B) anyone horned up out of their mind (and probably masturbating), or C) Someone who's down asf to talk about "What they'd do to *insert specific character here*"
Those types of mail are few and far between, but when they HAVE happened, it's been awesome!