Transport Rules

Transport rules (also called mail flow rules) are admin-defined rules in Exchange Online that act on every message passing through the tenant — incoming, outgoing, or internal. Each rule pairs a condition (sender domain, subject keywords, sensitive content, recipient group) with an action (add a disclaimer, redirect, encrypt, route through a connector, journal, append an external-sender warning, block delivery). Configured in the Exchange admin center → Mail flow → Rules. They're admin-scoped, not per-user — different from Outlook rules. Powerful but easy to mis-configure; broken transport rules are a common cause of mail-delivery problems.