Exchange Online Transport Rule

An Exchange Online Transport Rule (ETR), also called a mail flow rule, is a server-side rule configured in the Exchange admin center or via PowerShell that applies actions to email messages as they pass through Microsoft 365's mail transport pipeline. For email signatures, the action is `ApplyHtmlDisclaimerText`, which appends an HTML disclaimer or signature block to messages matching the rule's conditions (sender, recipient, message attributes). Because the rule runs at the server, the signature is applied universally regardless of which client the user sent from: Outlook desktop, Outlook on the web, Outlook mobile, Apple Mail, mobile native mail apps, third-party clients, even programmatic SMTP sends. The trade-off is that the user does not see the signature in their Sent Items folder unless additional configuration is added (such as a journal rule capturing the post-rule message). Transport rules also have limited variable expansion: built-in attributes like `%%DisplayName%%`, `%%Department%%`, `%%PhoneNumber%%` work, but anything not in the Active Directory schema requires preprocessing.