Lost access to old Microsoft hotmail after alias change

Hello,

I had an old Microsoft email address (hotmail) that I used to register my Microsoft account here. I have just logged in here using the new alias email address.

So, I added a new alias email address and made it primary. After that, I somehow removed or lost access to my old email address. Cant see it or make it primary.

Microsoft Authenticator on my phone still shows my old email on top but works for this new e-mail address.

However, external websites (for example Booking.com) probably still send login verification codes to my old email address,  which I no longer have access to. 

I need assistence with restoring access to my old Microsoft email address, or re-activating it as an alias, or getting official confirmation that the old address is permanently deleted.

Please advise on what can be done.

Thank you.