The existing /usr/sbin/ppp-off script should work just fine. The only changes you may wish to make are for the script to wait for any outgoing email currently being processed by sendmail.
In addition, you can create a script file that will be executed once the link has been terminated. This is stored in /etc/ppp/ip-down. It can be used to undo anything special that you did in the corresponding /etc/ppp/ip-up script.