Nary a clue as respects IMAP.
But if what you're seeing is similar to the Mail > Prefs > Accounts > Server Settings > Host Name(s) that I see with POP, they look like Internet addresses, in which case I'll guess that they're not case sensitive.
When I originally set up my POP account with RoadRunner, I was told that the correct setting was pop, but that didn't work. Apparently, the tech support person didn't know that it was case sensitive. When I changed it to POP, all was well. (Same for SMTP.)
