An SPF record is a small text file that tells ISPs who you are, and what domains you will send legitimate email from. You publish this file on your DNS server and more and more frequently, an incomplete or inaccurate SPF Record will cause your emails to be blocked by the recipients Internet Service Provider (ISP).
Your SPF record should include information about your domain and the domains of any third-party e-mail service providers you use.
How does it work?
Any time an e-mail is sent, the receiver’s mail server checks the domain name of the servers sending the message (hidden in the message headers). If it matches any of the domains listed in the sender’s SPF record, the mail is authenticated and delivered to the receiver. If there is not a match with the published SPF record or the SPF record doesn’t exist, the mail fails authentication and is not delivered.