I am jailbroken if this helps. If a cydia app can do this, mods, please move it to the hacking section.
It is annoying, as I often forget to open the mail app, and mail does not get sent until I do. As an example, I open photos and email one of the pictures. It sits is some kind of queue, and when I open up the mail app, it gets sent.
Can mail be sent without opening the mail app?