A useful mail-flow diagram shows decisions, not just product logos.
For each path, include:
- sending or receiving system;
- DNS/MX relationship;
- public and private handoff points;
- gateways;
- SMTP connectors or relays;
- TLS requirements;
- where SPF is evaluated;
- where DKIM is signed;
- where messages are modified;
- where logs and queues exist.
Draw inbound, outbound and application-relay paths separately when they differ.
Keep the diagram high enough level to share operationally without publishing credentials, private addressing or unnecessary security-control details.