You can find a server hostname by the IP address using telnet to port 25 (exim)

You can find a server hostname by the IP address using telnet to port 25 (exim):

1) Type: telnet <IP_ADDRESS> 25

2) Type: HELO [ENTER] Note: On some mail servers, you have to include a hostname to avoid an error 501, such as: HELO localhost [ENTER]
3) Note the server name in the response.
4) To exit, type: QUIT [ENTER]

$ telnet 127.128.129.100 25
Trying 127.128.129.100…
Connected to 127.128.129.100.
Escape character is ‘^]’.
HELO
220-somehost.localhost.local ESMTP Exim 4.69 #1 Thu, 17 Feb 2011 07:31:40 -0800
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.
250 somehost.localhost.local Hello [127.128.129.100]
QUIT
221 somehost.localhost.local closing connection
Connection closed by foreign host.