Outlook 2016 doesn't accept password or app password

Thanks for the feedback and your first impressions. I am not sure if you are utilizing the favorite folders in the New Outlook just yet. But the way I have it setup is it is syncing all of my accounts, and I add the inbox of each folder to my favorites (right click on the folder you want to add and select add to favorite).

We have introduced drag and drop reordering of the favorite list. This is the way I order them and then just quickly go through them in order that I prefer.

I will indeed pass on the full-on account re-ording to that part of the team as well.

I can not necessarily speak to any different third-party email applications since I personally don't use them deeply or work on their product teams. However, I can speak to Outlook. You should be able to continue to use any Outlook client that has multiple folders and mail in all of them and be able to sync quickly.

For any other third party client, IMAP is the main protocol and the standard that many email services use to sync.

At one time Apple did default to using Basic Auth when connecting to Outlook.com/Hotmail.com, so if you had this account setup for a while it would continue to be setup that way. There isn't anything behind the scenes that they just change, it will require a user to remove and readd the account.

When you are within the Apple Mail app and Adding a New Account, you should select Outlook.com as the email provider. Then login with your username and password.

We for sure do not want to cast anyone aside, and totally understand the frustration when mail is not syncing or things aren't working as expected. This is totally why I was recommending opening an in app support ticket from both the Mobile device as well as the Microsoft 365 Outlook client so that the support team can see the logs and see exactly how things are configured. Being able to see logs and details about config is going to be much more clear and concrete.