An “include” is an SPF mechanism that points to a domain to be queried when checking if the sending IP is allowed or not. If the sending IP is part of the “include” then it results in a match and SPF passes. 

For example if you have “include:_spf.google.com” as part of your SPF record and the originating IP of an email is a Google’s IP, it will result in a match as you have authorized google to send on your behalf and the sending IP is found inside of the “include” mechanism.

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