Transparent SMTP proxy

SMTP proxies are specialized mail transfer agents (MTAs) that, similar to other types of proxy servers, pass SMTP sessions through to other MTAs without using the store-and-forward approach of a typical MTA. When an SMTP proxy receives a connection, it initiates another SMTP session to a destination MTA. Any errors or status information from the destination MTA will be passed back to the sending MTA through the proxy.

SMTP proxies are commonly used to process and filter inbound and outbound email traffic.

SMTP proxies often serve as the initial, network-facing layer in an email system, processing SMTP connections from clients before forwarding data to a second layer of mail servers. SMTP Proxies often implement the first and/or only layer of defence in an inbound anti-spam filtering system, where they can analyze messages using a spam content filter or antivirus program, block or rate limit connections using DNS blacklists and reputation system, and load balance SMTP connections to prevent overloading of mail servers.

Because SMTP proxies do not store messages like a mail transfer agent (MTA) does, they can reject SMTP connections or message content in real-time, doing away with the need for out-of-band non delivery reports (NDRs), which are the cause of backscatter email, a serious problem in the Internet email system.

Certain SMTP proxies implement management (otherwise known as flow control), which can help to reduce damage to downstream mail servers resulting from spikes in traffic from malicious SMTP clients. management in the context of SMTP typically involves bandwidth throttling and/or introducing delays in SMTP command responses (this is also known as tarpitting). When slowed down, certain malicious sources of SMTP traffic such as spam bots tend to give up rather than continuing to deliver a full email message.

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Wikipedia