Exchange Online Mail Flow Decision Builder: Rules, Connectors and Sending Patterns for 2026

Exchange Online mail flow is not just about making email work. It is about deciding which messages should be trusted, blocked, routed, modified, quarantined or inspected. This guide gives you an interactive decision builder, recommended patterns for 16 real-world scenarios, a practical SMTP AUTH / HVE / Graph / ACS comparison, dangerous bypass rules to fix, connector review checklists, and the operational advice I use when reviewing mail flow in production tenants.