Anti-relay or anti-email replay (Open-Relay)

What is an email security anti-relay?

Definition of 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.

Glossaire Définition Anti-relais

Application at Altospam

Relaying is a technique that allows third parties to use your mail server to send often illegal e-mails. This abusive use of your server generates a problem of availability and quality of service, and also engages the responsibility of your company, whose identity has been usurped. ALTOSPAM prevents relaying, as ALTOSPAM servers are positioned between the sender and your mail server, meaning that the sender can no longer be reached directly via the Internet.


Example of an SMTP transaction ending with an anti-relay rejection:
z03:~# telnet 25
250 “HELO OK.”
mail from:
rcpt to:
550 “Relay forbidden!”


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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