Mail address not available

Hi,

I was asked to verify my email address with iCloud. Using the email address that it already had, i received the email message and entered the code that I received in the mail. I received the response:

This email address is not available. Choose a different address.

I got the same result when I tried to log in to my apple account, and after some security questions it asked me to verify my email.

I've been using the same email address with my apple account for decades. What is the problem?