Hi @Nguyen Khoa Van ,
Welcome to our Q&A forum!
Since you have mentioned that you are synchronizing on-premises Active Directory with Azure Active Directory with Azure AD connect, please clarify does this issue occurred after running the process?
If so, this issue may occur if there are duplicate SMTP proxy addresses in your local directory or in Microsoft Entra ID. To resolve this issue, you need to find and remove the duplicate addresses so that they are unique for each user.
you can use the Get-MsolGroup or Get-AzureADGroup cmdlets to find the objects that have the same SMTP proxy address. For example, you can run the following command:
Get-MsolGroup -SearchString email@example.com
Besides, AAD Connect has its add-in Troubleshooting Tool, which can help admin to troubleshoot configuration settings such as Object synchronization, hybrid deployment .etc. You can refer to: https://www.sikich.com/insight/troubleshooting-microsoft-azure-ad-connect-powershell/
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