To reject an email from foo@bar.com
===================================

[1] If you're root,

# vi /etc/mail/access

     foo@bar.com REJECT

# makemap hash /etc/mail/access < /etc/mail/access


[2] If you're not root,

$ vi ~/.procmailrc

LOGFILE=$HOME/Mail/procmail.log
VERBOSE=yes
PATH=/usr/bin:/usr/local/bin:/bin
SHELL=/bin/sh
FORMAIL=/usr/bin/formail
SENDMAIL=/usr/sbin/sendmail

:0 h
* ^Fromfoo@bar.com

-----------------------------------