RFC 1034 — Domain Names: Concepts and Facilities
Why This RFC Exists
RFC 1034, published by Paul Mockapetris in 1987, lays out the concepts and facilities of the Domain Name System — the distributed database that maps names to records. It is the foundation every other DNS and email-routing RFC builds on.
The CNAME exclusivity rule (§3.6.2)
A CNAME record makes a name an alias for another name. RFC 1034 §3.6.2 requires that if a CNAME exists at a name, no other record types may exist there — so the alias resolves unambiguously to its target.
Why it matters for email
If your root (apex) domain is a CNAME to a hosting provider (Heroku, Cloudflare, and others), you literally cannot add SPF (a TXT record) or MX records at that name — the CNAME rule forbids it. Authenticated sending from that domain is therefore impossible. The fix is a dedicated sending subdomain that can hold its own SPF, DKIM, and MX records.
Practical guide
For the hands-on setup, see Why you should use a subdomain for sending email.