An SPF record is a Sender Policy Framework record. It’s used to indicate to mail exchanges which hosts are authorized to send mail for a domain.

Sender Policy Framework records should follow specific syntax guidelines. In some cases your email provider may generate a ready-to-use string for you, but you can customize it yourself. You can use a third-party SPF Wizard or any other online tool to simplify the task.

NB! According to RFC 7208, "SPF implementations must limit the number of mechanisms and modifiers that do DNS Lookups to at most 10 per SPF check, including any lookups caused by the use of the "include" mechanism or the "redirect" modifier"". The mechanisms of: "include", "mx", "a", "ptr", and "exists"  count against the limit of 10 lookups. The "all, "ip4", and "ip6" mechanisms do not count against the limit of 10 since they do not require a DNS Lookup.