Email DLP inspects messages for information an organization does not want sent in a particular way.
Policies can look at content, labels, attachments, sender/recipient context and other signals. Actions might audit, warn, block, quarantine, encrypt or require another control.
False positives are an operational problem
A pattern that resembles a sensitive identifier is not necessarily sensitive data. Good DLP programs use context, testing, exceptions and incident review instead of turning on broad blocking rules and hoping for the best.
Know where enforcement occurs
A cloud mail platform, secure email gateway and endpoint DLP product can all make different decisions. Document which layer owns which policy.