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.
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.