Thanks for the details. Have you already added a new personal email alias to your Microsoft account and verified it under Your info at account.microsoft.com?
If the sympatico address is the primary sign-in and tied to legacy domain rules, Microsoft blocks changing it directly. The official steps are to add and verify a new alias, set it as primary, then remove the old one. If the system still shows the policy message and will not let you switch the primary alias, you need to contact Microsoft support to update it on their side so your sign-in and notifications move to your new email.