Postgrey

Postgrey: SMTP Proxy for Greylisting

Temporarily delays mail delivery to help mitigate spam, using greylisting to reject initial email delivery attempts and confirm legitimacy.

What is Postgrey?

Postgrey is an open source SMTP proxy program designed to help mitigate email spam. It uses a technique called greylisting to reject initial delivery attempts for emails from unknown servers. Legitimate email servers will retry delivery after some delay, while spam servers typically won't bother retrying.

Here's how Postgrey works:

• It sits between your mail server and the internet, intercepting all incoming SMTP connections.
• When an unknown mail server first tries to deliver an email, Postgrey temporarily rejects it with a 4xx error code.
• Legit email servers will retry delivery after 5-15 minutes per SMTP standards.
• Spammers usually don't retry, since they want to deliver quickly and move on.
• Postgrey remembers hosts that retry, passing their subsequent emails through normally.

So in essence, Postgrey buys you some time to see if an unknown sender is a real email server. By delaying spam for even a short period, it can reduce the load on spam filters significantly. And legitimate email just experiences a small one-time delay.

Some key features of Postgrey include:

• Lightweight, fast written in C
• Easy integration with common mail servers like Postfix, Sendmail, etc
• Automatic whitelist management as servers prove themselves legitimate
• Support for greylisting exceptions, whitelists, and blacklists
• Minimal impact on email delivery latency

So in summary, Postgrey is an SMTP greylisting proxy that helps filter spam by verifying that unknown sending servers attempt retry delivery per email standards.

Postgrey Features

Official Links