Tuesday, June 12, 2007

550 5.7.1 Unable to relay

Question has been asked one more time, about open relay and how we would make sense what was actually going on. I am not going to talk about how mail server becomes an open relay server, but I will try to make you understand how can a simple Telnet test be performed and how can we understand the outputs from this little test. A Telnet test involves establishing a Telnet session from a computer that is not located on the local network to the external (public) IP address of the Exchange server. You need to carry out the test from a machine at home, or from another office. Doing the test from a machine on your own network will produce useless results.

Start a command prompt.

Clicks start, run and type CMD

Type 25 (Substitute the IP address to your own IP address) this is your external IP address

You should get a response back similar to the following:

220 mail.server.domain Microsoft ESMTP MAIL Service, Version: 6.0.2790.0 Ready at

Type the following command in to the telnet windows:

Helo mydomain.com (note "Mydomain.com" can be anything but your real domain which your Exchange server is responsible for

250 mail.smtp25.org Hello []

mail from:Spammer@SpamKing.org

250 2.1.0 Spammer@SpamKing.org....Sender OK

Rcpt to:Spamking@SpamKing.com

550 5.7.1 Unable to relay for Spamking@SpamKing.com


Let's try to understand what happened in the lines above, We telnet into the recipient mail server on port 25 by typing

Telnet mail.recepientMailserver.org 25

We got 220 Form mail server with SMTP Banner

We said hello in SMTP way by typing below on the command line and HIT enter


We got 250 back from Recipient mail server ( 250 means everything is okay so far)

We said we will send you a mail and mail is coming from:
Spammer@SpamKing.org ( this is totally made up SMTP address, you could write anything without @ part or even <> this

We got 250 one more time (Recipient mail server says, sure we need to says next who we are sending this mail too, by typing below mail

Rcpt to:Spamking@SpamKing.com (here is the catch; the recipient mail server is Authoritative domain for SMTP name space which is @SMTP25.org. I am talking to this mail server and assuming I am someone which I am not actually, and I am telling the mail server accept the mail for a user Spammer@SpamKing.org if you paid attention. The server will or should think in this way. Why this person is asking me to accept mail for SMTP domain @SpamKing.org and the behalf of a user called Spammer@SpamKing.org, I am no t Authoritative for @SpamKing.org so I won't accept mail for this SMTP domain and issue following message

550 5.7.1 Unable to relay for Spamking@SpamKing.com, if you get 250 instead it means your server is OPEN relay, meaning the server accepting mails which the server is not Authoritative SMTP domain for.


Oz Ozugurlu


Anonymous said...

simple as that.. thanks

Daniel Piestrak said...

Thanks. This helped a lot.