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.

Uses

When SMTP proxies are placed on the outgoing network, they typically are used to intercept all SMTP connections to make sure that unauthorized e-mail, spam, e-mail worms, etc. are not sent from the network. This is common on internet connections provided by hotels, company networks, and some ISPs and is discussed in RFC 5068.

More commonly is when SMTP proxies are placed on the incoming network where they typically are used in the integration of anti-spam techniques into MTAs, the creation of e-mail hubs for load balancing, or to forward e-mail from a company-wide domain name to individual department mail servers. By using a proxy instead of the more common store-and-forward method of relaying e-mail, backscatter can be greatly reduced.

An advantage of SMTP proxies is that they work with any already installed mail server. If one decides to switch to a different MTA, they can still use the same SMTP proxy.

Certain SMTP proxies implement connection management, which ensures that no matter how big a spike in traffic coming from the...
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