Open-Relay
Anti-relay or anti-email replay
Definition: What is an email security anti-relay?
Anti-relay is a security system that detects and blocks e-mail from unauthorized or misconfigured relay servers. It can be used to prevent spam, email attacks and mail server abuse.
How does email security anti-relay work?
An anti-relay works by checking the email header to determine whether the originating mail server is authorized to send emails using your domain. If the server is not authorized, anti-relay can block the email or flag it as suspicious. Anti-relay can also use blacklists to block relay servers known to send spam or malicious content.
Examples
Example of an SMTP transaction ending with an anti-relay rejection:
z03:~# telnet c03.hornetsecurity.com 25
220 c03.hornetsecuritycom ESMTP
helo toto.com
250 “HELO OK.”
mail from: toto@toto.com
250 “MAIL FROM OK.”
rcpt to: dsfsf@dsjfslkdjf.com
550 “Relay forbidden!”
Applications
A simple test (see above) can be used to check whether a mail server (an MTA) is open-relay or not. Some blacklists automatically identify open-relay servers.
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