Note: this is meant as a self guide, so consider it as a draft!
I have spent a few months struggling to get Outlook 2021 working with my “Exchange” email domain! It works perfectly with Outlook 2016…
Interestingly, Windows 10-11 built in mail apps and Android (presuming iOS) Outlook app continue to work fine just using just your MS account sign-in.
A long time ago, it was possible to add custom domains to hotmail.com and live.com domains for free! Although MS stopped allowing new signups, the accounts that were already registered still continue to work (as per the original agreement).
Although this is the case, MS have seemingly decided to make this more difficult to use with later versions of Outlook (in order to move you to their paid Office365 service)!
Here is quick guide to adding a hotmail/live account (with custom domain) to Outlook 2021.
Note: It still uses the original SRV DNS records (Dont add “autodiscover.domain.cx” for risk of certificate errors).
You cannot do it though the outlook app wizard (which is run when starting the app for the first time), instead, visit the “Control Panel” -> “Mail (Microsoft Outlook)”
Create a “Default” profile.
Choose “Manual setup or additional server types”
Choose “Exchange ActiveSync”
Fill in your details and use “outlook.office365.com” as the mail server.
Now open the Outlook 2021 app and it should sync all your (many GB of email) as normal.