Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Jane,
Who is the email service provider for the email address you want to use that Microsoft is recognizing as an “organizational”?
Regardless, you can still add that address as security information so your 2FA verification codes are sent there instead of to the email address that will soon be discontinued.
Go to https://account.live.com > Security > Manage how I sign in > Email a code. Add the new email address there. Once it is added and verified, you can remove the old email address from your security information.
Even if Microsoft will not let you add the new address under Your info > Sign-in preferences as an alias, that does not prevent you from continuing to use the old email address as the username for your Microsoft account.
The sign-in address is essentially just an account identifier. Microsoft does not require that mailbox itself to remain active simply for you to use it as the username/alias. What is important is that you still have access to a working verification method for 2FA. That is why updating the email address under Security is the key step here.