Even if I only retrieve messages a few times a day, manually when near a WLAN access point, I would want to messages I send to be sent asap.
It would like to have an option to send ASAP, and keep sending until outbox is empty. OR some kind of reminder that there are unsent messages in the outbox, and the mail should be synchronized.
As it is, given the sometimes unreliable connection I have, I have to go check each outbox when I have sent urgent mails, just to make sure they have been sent.