RFC 2181 — Clarifications to the DNS Specification
ELI5: A follow-up that tightens the DNS rules: it spells out that an alias (CNAME) can't share a name with other records, and that mail servers (MX) and name servers must point at real hostnames, never at aliases.
Why This RFC Exists
Years of operational experience exposed gray areas in the original DNS specifications. RFC 2181 clarifies them precisely so implementations agree on edge cases.
CNAME and other data (§10.1)
RFC 2181 restates RFC 1034's rule unambiguously: a name owning a CNAME may own no other records. It also requires that MX and NS records point to hostnames with address records — not to aliases.
Why it matters for email
Because MX targets and the SPF/MX-bearing sending name can't be CNAMEs, mail must be sent from a name (a subdomain) that can hold real records.
Practical guide
For the hands-on setup, see Why you should use a subdomain for sending email.