O Avoid Setting Up an Open Relay

The Internet offers lots of resources for checking open relays; one of them is available at http://www.abuse.net/relay.html. You can easily find many more, or you can test your own server by sending a message from an untrusted IP address to an external domain:

<Verbatim listing 17> # telnet mail.example.org 25 Trying Connected to mail.example.org. Escape character is '*]'.

220 box.example.org ESMTP mail.example.org ; Mon, 9 Oct 2006 15:42:23 GMT HELO testclient.example.org

250 mail.example.org Hello testclient.example.org [], pleased to meet you mail from:[email protected]

250 2.1.0 [email protected]... Sender ok rcpt to:[email protected]

550 5.7.1 [email protected]... Relaying denied. </Verbatim listing>

Always ensure that your MTA is relaying to accepted domains and from trusted IP subnets. For an external client who needs to send messages through your mail server (i.e., roaming clients) you can use SMTP AUTH or TLS certificate validation to allow relay to external authenticated IP addresses.

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