I think you'll run out of file opens before the kernel starts to have serious problems. Regardless, there are some things you can try. Try setting the precedence of the mail to list or less. The allowed precedence settings in sendmail are:
Pspecial-delivery=100 Pfirst-class=0 Plist=-30 Pbulk=-60 Pjunk=-100
Add a header to your outgoing traffic setting the precedence of the message to list. If you're using majordomo it can be set in the $listname.config file in /var/lib/majordomo/lists/.