Hi Menno Demandt,
Have a Good day.
Thank you for posting to Microsoft Community, we are Happy to assist you.
Regarding to your description and according to inquiry related to create rule in a shared mailbox.
In the meantime, we have many methods to create a rule fo a shared mailbox:
· Method1:
Open the shared mailbox in OWA outlook on the web
Open the shared mailbox in a separate browser window
- Sign in to your account in Outlook on the web.
- On the Outlook on the web navigation bar, click your name. A list appears.
- Click Open another mailbox.
- Type the email address of the other mailbox that you want to open, and then click Open. Another Outlook on the web session opens in a separate window, allowing access to the other mailbox.
Then follow these steps to create a rule using outlook on the web:
- Select Settings

- Select Mail. Then under Mail, select Rules.
- Select Add new rule
Type a name for the inbox rule in the Name your rule box and then select a condition and action from their respective drop-down lists. You can add additional conditions or actions by selecting Add a condition or Add an action.
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· Method2:
Note: If you are not admin in your tenant, kindly contact your IT admin to perform the action.
if you are an admin, you can try creating rule via Windows PowerShell to troubleshoot if it caused by UI problems, for example:
New-InboxRule -Mailbox your shared mailbox name -Name "rule name" –FromAddressContainsWords "codition" -MarkImportance 1
For your reference: https://docs.microsoft.com/en-us/powershell/module/exchange/mailboxes/new-inboxrule?view=exchange-ps
Note: To use Windows PowerShell, please connect to Exchange Online PowerShell first.
• Note: Please understand that our initial reply may not always immediately resolve the issue. However, with your help and more detailed information, we can work together to find a solution.
Best Regards,
Microsoft Community Moderator