Troubleshooting Mailbox not found for this user error

Summary: When you open the properties of an Active Directory user remote mailbox, you may see the error: Mailbox not found for this user. This article explains how to resolve this error.

Applies to: Cayosoft Administrator 9.0.0 or later.

Resolution

In most cases, an Active Directory user account and its corresponding remote mailbox are linked via the UserPrincipalName (UPN) attribute. If the remote mailbox is not found, the most common cause is a mismatch between the UPN in Active Directory and the UPN in Microsoft 365 (formerly Office 365).

To resolve this issue, do the following:

Ensure UPN Consistency:

• Verify that the UPN of the affected user in Active Directory matches the UPN in Microsoft 365.

• If there is a mismatch, update the UPN in Active Directory or Microsoft 365 to align them.

Verify Object ID Mapping:

• Check the msDS-ExternalDirectoryObjectId attribute in on-premises Active Directory.

• Ensure that its value matches the ObjectId of the corresponding user in Entra ID.