A green process check is not enough.
Monitor:
- inbound/outbound SMTP reachability;
- queue age and depth;
- DNS resolution;
- MX changes;
- certificate expiry;
- DKIM/DMARC DNS availability;
- MTA-STS policy availability if used;
- TLS-RPT/DMARC report flow;
- disk/storage and system health;
- provider/API errors;
- authentication failures;
- abnormal outbound volume;
- representative end-to-end test messages where appropriate.
Keep synthetic tests controlled so monitoring does not become a source of noisy mail.