I quoted the entire related text, and there's no mention of either POP or IMAP.
I suspect that's Apple surmising that surely everyone uses IMAP.
If you're correct, it's pretty damned obnoxious of them!
Granted that it's the direction in which they're going, but the presumption is and always will be out of line.
But can you think of any reason why Mail Drop works only in IMAP accounts (if this isn't a bug)?
