https://www.kitterman.com/spf/validate.html?
https://www.dmarcanalyzer.com/spf/checker/
Maximum length of an spf record and splitting it into multiple strings:
https://support.agari.com/hc/en-us/articles/202952749-How-long-can-my-SPF-record-be-
SPF DNS Look up limits:
https://help.returnpath.com/hc/en-us/articles/222479888-How-can-I-avoid-SPF-failures-if-I-am-reaching-the-DNS-lookup-limit-