How licensing works for work and school accounts in the new Outlook for Windows

GlenB Thanks for the detailed comment. I believe I have addressed most of your questions below, please ask follow ups if there is anything I missed.

Consider a New Outlook deployment with Microsoft 365 Personal as the single primary/enable account.  Can a Microsoft Business Basic account be added as a secondary in that situation?  Is that a permitted deployment combination?

Yes, this will work and is a permitted combo. However, I question the term "deployment combination". Deployment makes me think you mean for a business or organization. We strongly encourage anyone who is a business to use Business Standard. That would be cheaper than Personal + Business Basic, has way more capability and value, doesn't require multiple accounts to be added to the app, is all managed through one admin center, etc. Business Standard is the configuration for any organization or business. The scenarios where you would use a Personal + BB are rarer, in situations where the end user is interested in Business Standard but doesn't have control of or the ability to influence the IT admin to make a licensing change. The two I see most commonly are: 

1. Most schools assign their students M365 Education A1 (web & mobile) licenses. That student may want the richness of the desktop apps, and so buys Personal on their own. Together, that allows them to use the A1 license with the full desktop M365 suite, including Outlook. 
2. Families who have setup a personal domain on Business Basic. But also want premium features tied to their personal Microsoft Account for things like OneDrive storage. 

It appears that New Outlook is intended to replace Windows Mail, which came free with Windows 10/11, and supported Business Basic accounts in both primary and additional configurations.  So users with a PC and only Business Basic could, if they choose, use Windows Mail as their client with no problem.  Is Windows Mail being forcibly deprecated, and if so, is there another path for Business Basic users other than upgrading to Business Standard? 

There is no other path. As Mail is retired, Business Basic users will either have to use the web and mobile M365 experiences that were included in their plan or upgrade to Business Standard if they wish to use the full suite of M365 desktop apps, including Outlook. 

Moreover, we note the presence of the "Try the New Outlook" button in Outlook 2021 Classic Desktop. 

Outlook 2021 is a perpetual, point in time purchase, not entitled to feature updates. You buy it and have the rights to it as long as it will work. We provide 5 years of Mainstream support, after which it will become a non-supported product. Typically, though our support ends, things keep working for some time. Outlook 2021 + Business Basic will continue working until whatever point Outlook 2021 stops working. At which point, the customer will either have to buy whatever next perpetual product is available (e.g. 202x) or subscribe to Business Standard. Business Standard is the best route, as it is cheaper and has feature updates. As for the toggle being visible there, I need to validate that with our team. That would definitely be a bug, it should not be on there. I'll follow up with the team to check that. 

Addressing your question of vision and direction. Broadly, the updates outlined in the blog above will handle a significant majority of the feedback we've received so far. We have the data on who our users are, what licenses they have, who is using what apps, etc. It would have been ideal to have these flows supported from Day 1, but as you see from the feedback portal you linked to there are also many other higher ranked items users and customers are asking for. So there is always a tradeoff.

We are intentional about the decision to build a unified, single email application for Windows. Focusing our entire team on one application will allow us to better serve our users and customers, improving the overall quality of our product and increasing the pace of innovation we can deliver. That is the goal. Plans like Business Basic were, and always have been, licensed for web and mobile app usage only. Frontline users cannot use Outlook desktop apps at all, even with a desktop license. All Office desktop apps on Windows or Mac (including Outlook), have always been available in our standard or premium tier plans, and will continue to be so. Businesses have all this information as they decide which combination of plans and pricing best meets the needs of their company. 

As Windows Mail is deprecated and replaced by Outlook, we have two options for those who today are using it with Business Basic. The most straightforward is to use the Outlook web experience that is included in their plan. This provides more capability and a better overall experience with M365. Or they can choose to upgrade to Business Standard for an additional cost.