Action Required: Enable multifactor authentication for your tenant

Question

We have just received a set of messages from MS asking us to enable MFA for all users in our tenant. The issue is that all active users have MFA enabled and we have one disabled user ( that has been converted to a shared mailbox) that apparently has no authentication methods and i think this is the reason for the message. We don't want to delete the user as it deletes the mailbox - what should we do in this case?

Thanks