Forking a new background processing thread when sending a message

On many email clients (such as Outlook Express, Outlook and Thunderbird) sending is done as a background process ~ allowing me to continue sifting through other emails, or begin writing a response whilst the sending processes. Postbox however, uses the same processing thread for the send mail template to send the mail - effectively not letting me do anything while my messages send. This becomes cumbersome if i have to respond to numerous individual messages.