What is SPF?
How SPF authorizes the services that send mail for your domain.
Start with the basics
SPF is a TXT record that declares which services may send email for your domain.
What to check
Publish one SPF record only. Include every provider that sends for you, then end it with an appropriate all mechanism after testing.
A safe next step
Avoid uncontrolled nested includes: receiving systems can stop evaluating SPF after too many DNS lookups.