Comment 2 for bug 810769

Jono,

This is available as a partially implemented feature, described briefly at http://kb.mozillazine.org/Thunderbird_3.0_-_New_Features_and_Changes#Send_in_Background Several people, including myself, have it enabled for a long time.

However, there is a lot of work that remains is https://bugzilla.mozilla.org/show_bug.cgi?id=511079 - Leverage Offline capabilities to make sending email appear faster (aka Send in Background, preference mailnews.sendInBackground)