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I do not have the numbers but significant amount of iOS users have Gmail or iCloud mail accounts. Mailbox was originally only for iOS and Gmail, later they have added iCloud, later they have added Android. Maybe in some time IMAP will be added as well. But it is sure as hell more viable to start with something that covers enough users to keep the lights going before trying to do everything.

I concur. I had to test a new IMAP Server for a Linux distro years ago, a lot of complexity.

POP is pretty darn simple though. I used to read mail, when away from home, using 'telnet mail.example.com 110'.