Transport rules have been in Exchange since the on-prem days. Microsoft keeps adding newer policy engines, but mail flow rules remain the workhorse for anything those engines do not cover. This article covers the baseline rules most tenants need, external sender tagging (custom vs native), disclaimers, security-focused patterns, encryption triggers, the AND/OR condition logic trap, rule priority and stop processing, where transport rules overlap with Purview DLP and Defender, PowerShell management patterns, limits and constraints, and a 14-point audit checklist.
