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-   -   Check For Open Relay Using Telnet (http://www.go4expert.com/articles/check-relay-using-telnet-t5984/)

pradeep 22Aug2007 18:27

Check For Open Relay Using Telnet
Behind the scenes, the commands sent to mail servers are simple text commands. These commands can be to sent to an email server manually via telnet. This is a quick way to test an email server to determine if it is an open relay.

First, determine the MX for the domain in question, run this command on command prompt:

nslookup type=mx go4expert.com

it should return something like:


Non-authoritative answer:
 go4expert.com  MX preference = 10, mail exchanger = ASPMX.L.GOOGLE.com
 go4expert.com  MX preference = 20, mail exchanger = ALT1.ASPMX.L.GOOGLE.com
 go4expert.com  MX preference = 30, mail exchanger = ALT2.ASPMX.L.GOOGLE.com
 go4expert.com  MX preference = 40, mail exchanger = ASPMX2.GOOGLEMAIL.com
 go4expert.com  MX preference = 50, mail exchanger = ASPMX3.GOOGLEMAIL.com
 go4expert.com  nameserver = ns1.go4expert.com
 go4expert.com  nameserver = ns2.go4expert.com
 ASPMX.L.GOOGLE.com          internet address =
 ASPMX.L.GOOGLE.com          internet address =
 ASPMX2.GOOGLEMAIL.com  internet address =
 ASPMX3.GOOGLEMAIL.com  internet address =

The first five lines tell you about the mail server (MX = Mail Exchange). In this case, ASPMX.L.GOOGLE.com( and ALT1.ASPMX.L.GOOGLE.com(

So, armed with this knowledge, in command prompt type


telnet 25
Server should respond with: 220

Server responds with: 250 OK
MAIL FROM:user@go4expert.com

Server responds with: 250 Address Ok.
RCPT TO:user@otherdom.com

Server responds with: 250 user@otherdom.com OK

Server Responds (or may not): 354 Enter Mail
Enter message, then on a new line,

The message should now be sent, if the server does not accept the message its not open relay. By modifying the MAIL FROM and RCPT TO lines, you can test for open relay.

rajkumar_singhalmca 25Aug2007 13:26

Re: Check For Open Relay Using Telnet
i am read this information but you cannot maintain that this service can be use by the l.a.n to w.a.n

pradeep 25Aug2007 14:26

Re: Check For Open Relay Using Telnet
You can, if you have a gateway to the internet!

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