Outlook. Seriously. It supports multiple accounts I believe (I only use my work exchange account so not positive), and it has scheduling of messages. The scheduling hasn't synced very well to the desktop version in my experience, but a lot of people have called it the best gmail client on iOS.
Although my primary OS for dealing with email is OS X and I see the OS X app is $130? Acceptable for a good Mailbox replacement, but a bit pricy for a gamble. :(
And "The scheduling hasn't synced very well to the desktop version in my experience" worries me a lot.
He means the Outlook iOS/Android app, which was a 3rd party app Microsoft bought (Accompli) and further upgraded by the people at Sunrise Calendar (also bought out) to make the app that it is today. Outlook for OSX, though, is still the classic Microsoft Office for OSX app, so it doesn't have any of those upgrades. The iOS app is my daily driver, and it works splendidly.
