Hi @CTuser ,
What's the type of your not supporting accounts in New Outlook?
As I know, new Outlook for MAC only supports certain types of accounts (M365, Outlook. com, Google) due to certain limitations, for more information, please refer to this official article: The new Outlook for Mac.
Also, Microsoft teams are indeed working on adding support for other account types in the future, you could vote this idea in uservoice, which may increase the priority for our engineers to create a possible option or feature that can be included in the future updates of the program.
For your requirement of those unsupported accounts now, it's suggested to temporarily go back to the Old Outlook for Mac. We understand this is inconvenient, but we are working hard to bring back support for this accounts as soon as we can.
Thanks for your understanding and hope that would be helpful to you.
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