In message "Diane
Poremsky [MVP]" was claimed to have wrote:

Why won't your isp allow you to use your email address in Outlook?

Many ISPs are starting to provide webmail-only email addresses.

It's a win-win for the ISP, they get to show ads to attempt to pull in a
bit more revenue, along with the reduced support costs of having to
support buggy mail clients and users who can't manage to enter server
information into their client and leave it alone.