Hi @David Kafrissen ,
From the description, seems like your users are having issues using shared mailboxes after switching to New Outlook. Given this situation, you can ask the affected users to manually toggle off the Try the new Outlook:
Meanwhile, you can also disable employee access to the new Outlook for Windows by following the article below:
https://learn.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/outlook-on-the-web/enable-disable-employee-access-new-outlook
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