About SPF records
SPF (Sender Policy Framework) specifies servers/hosts authorized to send email on behalf of a domain.
Consider the following illustration: 
Suppose mail.fazuh.com is sending an email to mail.google.com. In this case, the “Sending Email Server” is fazuh.com’s mail server, and the “Receiving Email Server” is google.com’s mail server.
After google.com receives the email from fazuh.com, its mail server queries fazuh.com’s DNS to check the SPF record and decide whether the “Sending Mail Server” is authorized to send email on behalf of fazuh.com, as specified on the SPF record.
If the “Sending Mail Server” is not specified in the SPF record, google.com either rejects the email, or marks it as spam. Otherwise, google.com accepts validated email.
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