Mail flow rules can inspect messages and take actions such as adding headers, redirecting, rejecting, moderating or applying disclaimers.
That power makes testing important.
Before enforcement
Write down exactly what should match and what should not. Test representative internal, inbound and outbound messages, including exceptions.
Watch interactions
Rules can overlap. Order, stop-processing behavior, exceptions and later changes can produce results that are hard to predict from one rule viewed in isolation.
Message modification
Rules that alter subjects, bodies or headers can affect downstream processing and, in some architectures, DKIM signatures.