A user has a personal Microsoft account (MSA) using their work email address (e.g., userid@example.com), which conflicts with the organization's M365 tenant where example.com is a verified domain. This causes "Work or school account?" prompts and prevents using the work email as the M365 sign-in.
- User has an alternate email address they control (e.g., Gmail, Yahoo)
- Admin access to M365 tenant