SPF / TXT Records Background
SPF records must be published as a DNS TXT Resource Record (RR). The use of alternative DNS RR types, including the SPF record type, that were formerly supported during the experimental phase of SPF were discontinued in 2014. [See RFC 4408, Section 3.1]
During the period when SPF was in development, requirements for assigning a new DNS RR type were more stringent than they are today. As a result, the SPF RR type was not widely deployed or supported across DNS servers and provisioning systems. The end result was that developers of SPF discovered it was easier and more practical to follow the TXT RR type for SPF.
The TXT RR type should be used for SPF records. The SPF record type is no longer part of the RFC standard & most newer mail systems will only support TXT records for SPF queries.
Adding an SPF RR type is not necessary, but may still be supported by older systems. If adding an SPF RR type, users should also create a corresponding TXT RR to ensure the record is supported & compliant with current RFC standards.