This extension is located under "Email → Authentication "

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Email authentication helps you combat junk mail. The following email options allow you to attach authentic information to your message, which will be used in automatic verification of incoming and outgoing mail.

Enabling these features will reduce the number of failed delivery notifications you receive when spammers spoof messages from your domain(s). These features also help prevent spammers from passing off unwanted messages as messages from your domain.

""SPF (Sender Policy Framework)** is an extension for the SMTP email sending protocol.)

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SPF allows the owner of a domain, in a TXT entry corresponding to the domain name, to specify a list of servers authorized to send email messages with return addresses in that domain.

Mail transfer agents that receive mail messages can query SPF information with a simple DNS query, thus verifying the sender's server.

SPF allows you to specify servers and IP addresses that are allowed to send mail from your domains. This feature is designed to block outgoing unwanted messages.