You know, a
Microsoft Account. An account required to access all sorts of MS services. For example, purchase Windows apps from the Microsoft Store, use Teams video-conferencing, Outlook.com email, play Minecraft, register Visual Studio, Xbox, etc.
The situation goes like so:
1. Many years past you sign up for an MS account using me@example-isp.com.au as your e-mail address, which becomes your account's username.
2. You change ISP or it goes out of business, and you no longer have that e-mail address.
3. You can change the associated display name and e-mail addresses (what MS term 'aliases') on your MS account, but you can never change the canonical username, which is forever stuck as me@example-isp.com.au. Even if example-isp.com.au no longer exists.
I did a bit of digging in case things have changed (it was several years ago I went through this), and according to
this you can log in with any one of your 'alias' e-mail addresses instead of the canonical username, so I suppose nowadays it makes no practical difference, but it still bugs me.