The issue is the "New" Outlook. This is not a SmarterMail issue.
When the New Outlook was initially released, it only worked for POP/IMAP connections to a few providers: Hotmail, O365, Gmail, and Yahoo! Other connections were said to be coming, but for the time being those were the only connection types supported. The New Outlook couldn't even connect to on-premises versions of Exchange. This is why we always told people to move to "Legacy" Outlook as that was more forgiving and worked across platforms, or to move to a different client altogether.
As the New Outlook has matured, it allegedly supports IMAP connections from third parties. (No word on POP, though.) However, people are still complaining that they are having issues connecting via IMAP. In addition, the larger issue is that, even when connecting to your mail account via a third party provider (like your own account that's hosted by your business, organization, or even by yourself), your information is passing through Microsoft servers. Here's a good article on what that means --
https://www.xda-developers.com/privacy-implications-new-microsoft-outlook/ -- and we'll have a blog post coming up about it as well.
So, if you have the ability to switch back to "Legacy" Outlook, take it. A better solution, however, is to move off of Outlook altogether and explore other clients. Our recommendation is to look at eM Client as it mirrors much of Outlook's functionality, but is much easier to use, is more reliable, and doesn't come with the data concerns that Microsoft does.