A rule that files invoices works all week, then stops on the weekend. Another rule never runs at all after switching apps. Almost every rules mystery in Outlook is one distinction - server side versus client only.
Outlook has always had two kinds of rules. Server side rules live in the Exchange or Microsoft 365 mailbox and run day and night, even with your PC off. Client only rules live in classic Outlook on your PC and run only while that program is open - because their actions, like moving mail into a local PST folder or running a script, can only happen on your machine. The new Outlook works with server side rules and its own web style rules management. This page clears the confusion that breaks filing systems during a switch.
Example panel - the action decides the type, not the name you gave the rule.
A rule is server side when everything it needs exists on the server: conditions on sender, subject or recipient, and actions like move to a mailbox folder, mark as read, flag, categorise or forward. These rules are stored in the mailbox itself. They run when mail arrives, whether Outlook is open, closed, or not even installed. You can manage them from classic Outlook, the new Outlook, or Outlook on the web, because all three are editing the same server list. If your filing system is built purely from these, it survives any app switch untouched.
Classic Outlook adds actions the server cannot do: move or copy into a local PST or data file, run a script, start an application, display a desktop alert with local specifics, print automatically, or reply using a local template file. Classic labels such rules client only, and they fire only while that copy of Outlook is running. The trap is silent failure: the rule exists, nobody disabled it, but the PC was off or the user switched to the new app - which does not run classic's client only rules at all - and filing quietly stops. Gaps in a folder that should auto file are the signature symptom.
Your server side rules carry over automatically because they were never in the app - they are in the mailbox. The new Outlook also offers rules management for those server rules, with a simpler action set than classic's full wizard. Classic's client only rules do not migrate and do not run. Some users open the new app, see their main rules present, and assume everything moved - then discover weeks later that the PST filing rule has not run since the switch. The fix is a rebuild, not a recovery: recreate the logic with server actions where possible, or accept that this desk's filing rule keeps it on classic.
Many client rules exist only because they grew historically. The invoice rule that files to a PST folder can often become: server rule files to a mailbox folder named Invoices, and once a quarter someone archives that folder to PST in classic. The alert script can become a server forward to a Teams channel mail or a Power Automate flow. The print rule is the stubborn one - server side printing does not exist; desks that auto print orders or gate passes on arrival genuinely need classic running, which is a legitimate architecture, not a failure to modernise.
Both apps suffer when a mailbox carries forty overlapping rules nobody remembers writing. Rules run in order, stop processing flags change outcomes, and two rules can fight over the same mail. During any app change, do a rules audit: export or screenshot the list in classic, delete the dead, merge the duplicates, and confirm the order of what remains. A clean server rule set is the most portable thing in your whole Outlook setup - it follows you to every app and the web, working while you sleep.
See both Outlook apps side by side in the full video - every difference shown on a real PC, step by step.
Watch Full VideoOnly server side rules do. They live in the mailbox and run on arrival. Client only rules need classic Outlook open on your PC.
Server side rules yes, because they are stored in the mailbox. Client only rules from classic do not move and do not run in the new app.
In classic Outlook's Rules list, client rules are marked client only or on this computer only - usually because they touch a local file, script, printer or local template.