I have a user with an iOS device who can receive email, but is unable to send/reply to email. (iPhone 4s, iOS6, AT&T, 4G) User status in Monitor shows "Synced" and I can find no other errors. I don't know if this is only this user, or only iOS devices in general - testing with Android devices works as expected. I'm not sure what this implies - that one can receive but not send/reply. The fact that they can receive suggests communication with the server is OK.