Hi @Paul Winward ,
According to your description, the Online Archive folder can work fine on your web mail. So, we could infer that the issue is more related to your Outlook for Mac client.
As I know, Microsoft has released the new Outlook for Mac recently. Are you using the new Outlook for Mac now? If this is your case, I'm afraid that your issue might be normal. As the official document mentions, features like Online archive and others in this screenshot have not been available in the new Outlook for Mac yet. It is suggested for you to continue to use the old Outlook experience for now. (You could click the New Outlook button again to switch back.) Please wait for the update of new Outlook for Mac to support more features before switching to the new Outlook.
If all these mentioned above is not your case, please provide me with more information related to your issue for further research.
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