When queried for the SPF RR but there is none, the DNS server should return
the TXT RR in the additional section. In this way only one DNS query would
be required to retrieve the sender policy (SPF, DK, SES, IIM, etc.).
Following should be avoided:
- get the SPF RR from example.com to get the SPF record,
- if there is none, get the TXT RR from example.com to get the SPF record,
- if there is none, get the SPF RR from _domainkey.example.com,
- if there is none, get the TXT RR from _domainkey.example.com,
- if there is none, get the SPF RR from _ses.example.com,
- if there is none, get the TXT RR from _ses.example.com,
- if there is none, get the SPF RR from _iim.example.com,
- if there is none, get the TXT RR from _iim.example.com,
- and so on ...
Roger