These are the most serious sendmail security and reliability problems admitted by Eric Allman, from sendmail 8.6 in 1993 through sendmail 8.8.7 in 1997. Unattributed quotes here are from Allman's release notes. Any user on the Internet can force bounces for messages sent from the sendmail host to any selected target. I wouldn't be terribly surprised if new security holes in sendmail are found less frequently now. It's undergone some stringent code reviews.
Any local user can force a queued message to bounce.