Outbound filtering is not only a deliverability feature. It limits damage when a mailbox, API key or application is compromised.
Useful controls can include authentication, rate limits, anomaly detection, recipient limits, malware/content controls and rapid credential revocation.
Baselines matter
A payroll application sending 200 messages at 9 AM behaves differently from a mailbox suddenly sending 20,000 messages overnight.
Build alerts around the expected use of each sending identity rather than one global number.