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

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.