I don't remove the message frame after sending.  I write a lot of
email and find that the time spent rebuilding new frames is a
waste.  However, I believe you can use message-sent-hook to either
delete or iconfiy the frame.  Just make sure to select the right
frame for deleting or iconifying :-)

hth,
-- 
Dave Goldberg
Post: The Mitre Corporation\MS B325\202 Burlington Rd.\Bedford, MA 01730
Phone: 781-271-3887
Email: dsg@mitre.org