Microsoft recently renamed a majority of their products yet again because if it wasn't confusing enough already, it's even more confusing now! "Microsoft 365"

Microsoft recently renamed a majority of their products yet again because if it wasn't confusing enough already, it's even more confusing now! "Microsoft 365" now includes all of Windows 10, Office 365 (Office on PC) and Azure. This is more geared towards the home market. Then, there's "Office 365 for Business" which as the name implies is more for business/corporate customers. It looks like your custom domain is using the old Outlook.com/Hotmail architecture, which already supported +<tags> in email addresses - while Microsoft's corporate offering (running on a cloud based Exchange Server) still doesn't, and they're pushing support for it out later this year.