You can use the access file to generate messages. For example, I got sick of spammers hitting an old email account of mine (firstname.lastname@example.org) so in my access file I have:
email@example.com ERROR:550 Mailbox disabled for this username
I re-generated the access db with the following command. That email address no longer receives mail and the sender gets the message I have set.
makemap -v hash /etc/mail/access < /etc/mail/access
To use the access file you need to have it enabled in sendmail.cf. You can enable it by adding FEATURE(`access_db')dnl
to sendmail.mc and re-generating sendmail.cf.