RFC 1912 — Common DNS Operational and Configuration Errors
ELI5: A 'common mistakes' field guide for DNS. One classic error it warns about: using a CNAME on a name that also needs other records, which quietly breaks mail.
Why This RFC Exists
RFC 1912 is a practical checklist of the configuration errors operators make most often — the kind that pass casual testing but break in subtle ways.
The CNAME mistake (§2.4)
It specifically warns against using a CNAME on a name that also needs other records, and notes that MX (and other) records must not reference CNAMEs. This is exactly the trap that prevents email authentication on a CNAME'd domain.
Practical guide
For the hands-on setup, see Why you should use a subdomain for sending email.