[misterdata]

Microsoft appears to do it as well with Office 365 and Azure (and related services).

They appear to distinguish two types of accounts ('Live ID' type accounts for personal use, and 'Work' accounts) and it is possible for a single e-mail address to refer to both. When I enter my work email, I get to choose between 'use my work account' (which exists in Azure AD) and 'use my personal account' (which some years ago I registered as Live ID), then get the password prompt.

Actually their implementation is a bit annoying, because the password field in some cases is already visible before the choice between personal/work is presented - as soon as you tab out of the username field, you get the choice and you have to type your password afterwards.