Add a custom domain

Adds a new custom domain to your account and returns the DNS records you must configure at your registrar before verifying.

Plan required: Growth ($49/mo) or Enterprise ($149/mo).

Workflow:

1. POST /v1/custom-domains — add the domain, get DNS records.
2. Add the two DNS records (MX + TXT) at your registrar.
3. POST /v1/custom-domains/{domain}/verify — confirm propagation.
4. POST /v1/inboxes with any address @yourdomain — start receiving mail.

This call is idempotent — if the domain is already added it returns the existing entry with 200 instead of 201.

Limits: Maximum 10 custom domains per account.