Outlook rules and automation
Client-side rules, server-side rules, and Power Automate — the three ways to automate email handling in Microsoft 365.
There are three different "rule" systems for automating email in Microsoft 365. They look similar, behave differently, and have very different scopes. Knowing which to use saves a lot of frustration.
1. Server-side rules (Exchange rules)
These rules run on Exchange Online itself, against the mailbox, whether or not Outlook is open. They're stored on the mailbox and persist across devices. Triggers and actions include:
- Move messages by sender, subject, content.
- Forward or redirect (admin policies often disable external forwarding).
- Delete or flag.
- Mark as read.
- Apply categories.
Server-side rules are the right choice for almost all per-user automation. They're configured in Outlook (any client) and surface uniformly across devices.
2. Client-side rules (classic Outlook only)
Some rules run only inside the classic Outlook for Windows application. They depend on Outlook being open and signed in to fire. Typical triggers that force client-side rules: playing a sound, running a script (legacy VBA), starting an application. Client-side rules are increasingly rare and not supported in the new Outlook or on the web.
If you have users on the new Outlook, client-side rules don't run — review them before migration.
3. Power Automate
For anything beyond simple per-message handling, Power Automate is the right tool. Triggers include "When a new email arrives," "When an email is flagged," "When an email arrives in a shared mailbox." Actions span hundreds of connectors: post to Teams, create a Planner task, file to SharePoint, call an HTTP API, send to a ticketing system.
Power Automate also works on shared mailboxes, which Outlook rules don't always handle gracefully.
Transport rules (admin-level)
Don't confuse user-level Outlook rules with transport rules (mail flow rules) in the Exchange admin center. Transport rules run on every message passing through the tenant — they're an admin-scoped tool for things like adding disclaimers, redirecting to legal hold, blocking by content, or appending external-sender warnings. They aren't a substitute for per-user rules.
When to use which
| Need | Tool | | --- | --- | | Move newsletter mail to a folder | Server-side rule | | Create a Planner task when a project email arrives | Power Automate | | Add a legal disclaimer to all outbound mail | Transport rule | | Forward urgent messages to a phone | Server-side rule (with caution) | | Process emails into a SharePoint list | Power Automate |
Start with server-side rules. Reach for Power Automate when the logic crosses into other apps.