I am seeing a problem where a few minutes after the first 5-minute auto-save, the session is terminated and message composition is interrupted, kicking the user back to the start screen.
It seems to be ignoring my session_lifetime, which I changed to 90 (minutes) instead of the default 10.
I don't think it auto-saves before terminating the session, either. [Now that's user hostile!].
At any rate, commenting out these lines (effectively disabling session timeout) seems to be a temporary workaround.
//$message = show_message('sessionerror', 'error');
//rcmail_kill_session();
-- Guy