What is an SPF record?
An SPF record is a single DNS TXT record at the root of a domain. It starts with v=spf1, lists every service allowed to send as you with include:, ip4: and ip6: terms, and ends with an all term that says what to do with everything else. A domain gets exactly one.
Reading the result
The output is the record to publish as TXT at the root (@), with an estimate of the DNS lookups it costs out of 10. ip4: and ip6: terms cost nothing. Each include costs at least one, plus whatever it references in turn, so for a custom include the estimate is a floor. Duplicate includes are removed for you.
Mistakes it warns about
Too many includes
A mail provider plus a handful of sending tools can pass 10 lookups between them. Past the limit, receivers return permerror and SPF fails for all of your mail.
A second record instead of a merge
Adding a new v=spf1 record for each tool creates two records, which is a permerror. Every sender goes into the one record.
Includes for tools you dropped
Each one still costs lookups and keeps authorising another company's servers to send as you. Remove the include when you cancel the service.
Flattening includes into IP lists
Replacing includes with the addresses they resolve to saves lookups until the provider changes its IPs. Mail from the new ones then fails SPF, and nothing warns you.