Gmail adds support for "background send" on desktop
April 26, 2011
Fellow iOS Gmail users has already enjoyed the feature for some time, and apparently it works well enough for Google to let you turn it on yourself.
Still seems a bit shaky re. file uploads in the background - you need to have the e-mail (and any attachments) “finished” before pressing send for the backgrounding to work flawlessly. Still, it’s a step in the right direction - asynchronous operations ftw.