Sender Policy Framework

In many organizations multiple incoming mail servers (MTAs) are used, for example:

• A backup mail server that accepts mail and delivers it to the primary mail server
• A mail server that handles only incoming and outgoing mail, and not mail sent within the company
• A mail filtering (anti-spam) service that filters mail before it reaches the organization's mail server

In these scenarios only the initial ("border") MTA can check SPF status of a message. Otherwise the internal MTA would see the incoming connection coming from the border MTA.

Further, if the message was stopped at the border MTA instead of being accepted and forwarded internally, the internal MTA would not need to spend time processing the message.

For more information, see the Common Receiver Mistakes FAQ.