Definition
SPF is an email authentication protocol where you publish a DNS TXT record listing the IP addresses and hostnames authorized to send email from your domain. When a receiving server gets an email claiming to be from your domain, it checks your SPF record to verify the sending server is on the approved list. Without SPF, anyone can spoof your domain.
Example
v=spf1 include:_spf.mailchimp.com include:sendgrid.net ~all