
Explanation:

According to the Microsoft SC-300: Identity and Access Administrator Study Guide and the Microsoft Learn module "Implement and manage synchronization with Microsoft Entra Connect", when Microsoft Entra Connect synchronizes user objects between on-premises Active Directory (AD) and Microsoft Entra ID (formerly Azure AD), each user must have unique proxyAddresses attributes across the directory.
The proxyAddresses attribute represents all the email aliases (SMTP addresses) associated with a user. The synchronization process verifies these values to ensure they are not duplicated among multiple user objects. If two or more objects in AD share the same SMTP address, the synchronization process generates an error, typically reported as a "Duplicate Attribute" conflict.
In the provided scenario:
* User1 (proxyAddresses:
[email protected],
[email protected])
* User2 (proxyAddresses:
[email protected],
[email protected],
[email protected])
* User3 (proxyAddresses:
[email protected],
[email protected])
After updating the proxy addresses:
* User1 #
[email protected]* User2 #
[email protected]Because
[email protected] is now used by both User2 and User3, the sync process detects a duplicate SMTP address. This causes synchronization errors for User2 and User1 if any other duplicates exist.
User3, with unique attributes, synchronizes successfully.
The SC-300 reference under "Manage synchronization errors using Microsoft Entra Connect Health" confirms:
"If two or more objects contain the same value for attributes marked as unique (such as proxyAddresses or userPrincipalName), Microsoft Entra Connect synchronization fails for those objects until the conflict is resolved."