To make Microsoft 365 DLP policies work like Exchange transport-rule based DLP, a January change will switch evaluation of sender conditions away from envelope information to message headers. Although this change might seem to be something beloved of email geeks, it’s actually an important update for organizations who want to move away from ETR-based DLP to Microsoft 365 DLP policies.
https://office365itpros.com/2021/12/20/microsoft-365-dlp-switches-envelope-header-sender-evaluations...
