You may want to go down to the _ns_sendmail level or lower. ns_sendmail is a quick hack to let you do simple stuff easily, but isn't a full implementation of SMTP possibilities via its mapping of header information strategy.
On the other hand, ns_sendmail is built of pieces which build the socket, establish the SMTP connection, and send off the message with the appropriate SMTP headers.
You could plug into this without much trouble. OACS 4 packages like acs-mail will before long and bypass the ns_sendmail shortcut entirely.
