Error: The remote server returned an error: (401) Unauthorized.

401 Unauthorized (Unauthorised): The login details used to access the mailbox are invalid, or the login details specified do not have access to the configured mailbox, or you may have over utilised resources on the email server.


Invalid Credentials

This error is mostly caused by the user name and password specified not allowing access to the mailbox. If using admin credentials it is also posible that the admin credential may be incorrect , if this is the case all mailboxes within the project will fail with error 401.

Please verify if the user name and password are correct by attempting to log in to webmail. If you are connecting using administration level access, make sure to log in to the mailbox using administrator username and password because this is how Migration Monster will access the email systems.

If you have verified that the credentials are correct, try re-entering the credentials into Migration Monster as sometimes hidden space characters can cause problems.

Waiting for password reset

Check to make sure the account you are using to authenticate is not waiting for a password change by logging in to webmail/office 365 using the account details.

User Principal Name does not match primary email address (for on premise server)

If using email address format for user login, make sure that the UPN (User Principal Name) matches the primary email address for logging in.

For instructions on how to setup a UPN on a Windows server please see the link below. In most cases setting up a UPN has no affect on your current systems or login however, please check with your systems administrator.

Login Name Format issues

Depending on the email server configuration different login formats may be required for example, a full email address like , WindowsServerDomain\username or Usename by itself for example JohnSmith

When using the format domain\username, the username is truncated to 20 characters. If the username is longer than 20 characters, enter only the first 20 characters of the username.

Insufficient Resources

If you have eliminated user login issues from your investigation it is also possible that insufficient resources could be the cause.

If migrating to or from Office 365 and using as the destination/source URL to connect then try using instead. We would also suggest trying to connect again outside office hours when the Office 365 service is less heavily used.

Issues like poor CPU performance, low memory levels, high disk utilisation and network issues can all generate this error.

 Poor internet bandwidth can also cause this issue. In this instance we suggest reducing the number of concurrent migration allowed within the project configuration (by clicking the edit option against the relevant project on the projects (Home) page.


Insufficient user permissions for both user level accounts or administration level accounts may cause this error. Check the permission levels with your email server administrator.

Please also see our knowledge base articles in regards to admin permissions and impersonation.

Testing access

The following Microsoft provided tool is very useful for testing access to mailboxes. 

  • Go to
  • If using Office 365, click on the Office 365 tab.
  • Select Service Account Access (Developers) and the click Next.
  • Enter the mailbox email address.
  • Specify the service account user name (if using admin credentials on the connector, enter the exact same user name).
  • Specify the service account password (if using admin credentials on the connector, enter the exact same password).
  • Check Specify Exchange Web Services URL and specify the URL.
  • If using admin access within Migration Monster select impersonation.
  • Check Ignore trust for SSL.
  • Click perform test.
  • View displayed results