Duplicate Mail User Objects Created for Guest Accounts

The February 2025 EX1015484 incident explains why mail user objects with duplicate SMTP addresses are created for guest accounts. That’s a problem because Exchange Online can’t route messages to objects with duplicate email addresses. Fortunately, you can find out if any duplicates exist in your tenant with some PowerShell. It's a good opportunity to remind ourselves of the relationship between Entra ID guest accounts and Exchange Online mail user objects.

https://office365itpros.com/2025/03/28/ex1015484-problem/