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3AVA Mail

Subdomain vs root domain

Recommendation

Section titled “Recommendation”

Send from a subdomain like mail.acme.com or notifications.acme.com. Not from acme.com.

This is the default for almost every serious sender. Resend, AWS SES, SendGrid, Postmark — all recommend the same.

Why

Section titled “Why”

1. Reputation isolation

Section titled “1. Reputation isolation”

Your acme.com root probably gets used for personal mail ([email protected], [email protected]). If you blast 50,000 marketing emails from acme.com and a chunk get marked as spam, that complaint rate damages all mail from acme.com — including the founder’s email to investors.

A subdomain quarantines marketing traffic. If mail.acme.com gets flagged, acme.com is unaffected.

2. Different DMARC policies

Section titled “2. Different DMARC policies”

You can run a strict p=reject on the root (rejecting any unauthenticated mail claiming to be from acme.com) while running p=quarantine on the subdomain (more permissive while you build reputation).

3. Per-purpose subdomains

Section titled “3. Per-purpose subdomains”

Once you have one, splitting into more is easy:

  • transactional.acme.com — receipts, password resets, magic links
  • notifications.acme.com — product alerts
  • news.acme.com — marketing
  • bounce.acme.com — VERP envelope (separate so bounces don’t affect main reputation)

Each gets its own warm-up curve and reputation. A marketing complaint surge doesn’t break transactional.

4. Inheritance, not penalty

Section titled “4. Inheritance, not penalty”

A subdomain inherits a small positive signal from the root domain’s age and DNS history (especially MX records). It does not inherit the root’s bad reputation if the root has any. This means a fresh subdomain on a 10-year-old root domain warms faster than a fresh standalone domain.

When the root is fine

Section titled “When the root is fine”
  • Tiny volume (< 100/day) and you’re sure no marketing will ever go from there.
  • One-person sender — same person, all mail, no team.
  • You’ve already been sending from the root for years and it’s warm.

How to migrate from root → subdomain

Section titled “How to migrate from root → subdomain”
  1. Add the subdomain in 3AVA Mail.
  2. Configure DNS records for the subdomain (separate SPF, DKIM selector, DMARC).
  3. Update your from address in code: [email protected] → [email protected].
  4. Send a small batch from the new subdomain to confirm placement.
  5. Cut over.
  6. Leave the root domain DNS records in place — don’t delete its SPF/DKIM, or you’ll cause SPF failures for any in-flight mail.

DMARC alignment note

Section titled “DMARC alignment note”

For DMARC to pass with aspf=relaxed (the default), the SPF return-path domain must align with the From domain at the organizational level. mail.acme.com and acme.com align — you’re fine. They don’t have to be identical.