List every organizational domain and subdomain used for email.
For each, record:
- receives mail?
- sends mail?
- MX;
- SPF;
- DKIM selectors/providers;
- DMARC;
- return-path/bounce use;
- MTA-STS/TLS-RPT;
- responsible owner;
- vendors;
- retirement date if temporary.
Include domains used only by applications or marketing systems.
A domain with no owner is where stale SPF includes and forgotten SaaS senders tend to live.