SPF is a type of TXT record. Sometimes,some domain has several TXTs. For example, carrefour.com has about 18 different TXT records,when the mail from this domain,spam check result is MISSING SPF,but actually,it is exist,some other domain that have 4-7 different TXT records also have such issue.

So the software needs to get all txt records and search the key word: v=spf  (v=spf1........ or v=spf2.0/pra.........) to check whether the SPF record is exist.