It is easy.
$ /usr/sbin/sendmail -f dustin@dustincurtis.com
dustin@posterous.com
Subject: hi
Spam spam spam
^D $ /usr/sbin/sendmail -f dustin@dustincurtis.com
dustin@posterous.com
Subject: hi
Spam spam spam
^DOne quick whois lookup, and I found the email he was likely sending from. His site has another email listed, so I did a little digging.
$ telnet mailhost.com 25
HELO myserver.hostname.com
MAIL FROM: dustin@dustincurtis.com
RCPT TO: post@posterous.com
DATA
Subject: Hello
spam spam
.
QUIT