telnet mailserver.foobar.com 25
Trying 10.0.0.1...
Connected to mailserver.foobar.com.
Escape character is '^]'.
220 mailserver.foobar.com ESMTP Sendmail 8.11.2/8.11.2; Sat, 31 Mar
2001 13:07:14 -0500 (EST)
ehlo evil.com
<-- Note: The HELO/EHLO is not required for many mailservers
250-mailserver.foobar.com Hello badguy.somewhere.com [10.2.1.1], pleased
to meet you <-- Note, the REAL name and
250-ENHANCEDSTATUSCODES
IP are in the Hello reply
250-8BITMIME
250-SIZE
250-DSN
250-ONEX
250-ETRN
250-XUSR
250-AUTH LOGIN DIGEST-MD5 CRAM-MD5
250 HELP
mail from: blaw@something-made-up.com
250 2.1.0 blaw@something-made-up.com... Sender ok
rcpt to: blaw@somewhere-else.com
550 5.7.1 blaw@somewhere-else.com... Relaying denied <--
This shows that the mailserver is not an open relay
quit
221 2.0.0 mailserver.foobar.com closing connection
Connection closed by foreign host.