Mail Contact showing as UserMailbox in Exchange Online

Hello, Sivachandran_Palani 

welcome to the Microsoft community, my name is Recep I'll be happy to help you today.

If you are facing an issue where a mail contact created in Exchange On-Premises is syncing in Exchange Online as a "User Mailbox" instead of a mail contact, there are a few steps you can take to troubleshoot and resolve the problem. Here's a step-by-step guide:

Check the Recipient Type of the Object:

In Exchange Online PowerShell, run the following command to check the recipient type of the object:

Verify the Mail Contact in On-Premises Exchange:

In your on-premises Exchange environment, run the following command to verify that the mail contact is indeed created as a mail contact.

Check Sync Status:

Ensure that the Azure Active Directory (AAD) Connect sync is running successfully and has synchronized the changes from on-premises to the cloud. You can check the synchronization status using the AAD PowerShell module.

Force Synchronization:

You can force a synchronization from the AAD Connect server. Open a PowerShell prompt on the   server where AAD Connect is installed and run the following commands:

Examine Attributes

Check the attributes of the mail contact in both on-premises and online environments. Ensure that the necessary attributes such as RecipientType, RecipientTypeDetails, and RemoteRecipientType are set correctly.