Não é realmente correto que o tipo SPF
RR seja o padrão mais novo (no contexto do comportamento do SPF desejado). A fase experimental da especificação SPF teve um novo tipo de registro atribuído, mas o caminho da migração não era claro e já foi abandonado.
A versão atual da especificação do SPF declara especificamente:
SPF records MUST be published as a DNS TXT (type 16) Resource Record (RR) [RFC1035] only. The character content of the record is encoded as [US-ASCII]. Use of alternative DNS RR types was supported in SPF's experimental phase but has been discontinued.
In 2003, when SPF was first being developed, the requirements for
assignment of a new DNS RR type were considerably more stringent than they are now. Additionally, support for easy deployment of new DNS
RR types was not widely deployed in DNS servers and provisioning
systems. As a result, developers of SPF found it easier and more
practical to use the TXT RR type for SPF records.In its review of [RFC4408], the SPFbis working group concluded that its dual RR type transition model was fundamentally flawed since it
contained no common RR type that implementers were required to serve
and required to check. Many alternatives were considered to resolve
this issue, but ultimately the working group concluded that
significant migration to the SPF RR type in the foreseeable future
was very unlikely and that the best solution for resolving this
interoperability issue was to drop support for the SPF RR type from
SPF version 1. See Appendix A of [RFC6686] for further information.The circumstances surrounding SPF's initial deployment a decade ago are unique. If a future update to SPF were developed that did not
reuse existing SPF records, it could use the SPF RR type. SPF's use
of the TXT RR type for structured data should in no way be taken as
precedent for future protocol designers. Further discussion of
design considerations when using new DNS RR types can be found in
[RFC5507].
Como um sidenote, havia também um registro de ID do Remetente (infelizmente chamado de "spf2.0" apesar de ser uma especificação diferente) em seu exemplo, as regras para esse tipo de registro ainda são experimentais e corresponde à versão experimental da especificação SPF , nenhuma atualização foi publicada.