BulkMail would benefit greatly from having an 'auto-send' configuration setting. Doing so would have all mail sent out the moment the mailbox is viewed (MAIL_SHOW in-game event). Other addons such as PlainAutoMail feature this by having the user hold shift -> right click on the mailbox.

e.g.

if event == "MAIL_SHOW" then
	if auto_send.enabled and IsShiftKeyDown() then
		-- Send Mail Here
        end
end