YOu should modify ns_sendmail to take an extra argument, a "sender" address in addition to the "from" address. The current implementation uses the "from" address for both the mail "envelope" as well as the "from:" header. This makes it impossible to do reasonable things like automated bounce checking, error reporting, etc.
The "envelope" address is the email address sent in the SMTP "MAIL FROM" command. This is the email addresss that is used by the mailer at the other end to send error messages back to.
For a proper spam system, this should be set to some magic encoding
of the spam id or destination user id, so you can automatically collect and parse what email addresses are bouncing.
See http://www.arsdigita.com/asj/mime/
