Go to Exchange System ManagerServersYourserverFir­st Storage
GroupMailbox, on the right pane, you can see the Mailbox name is still
being remained as the original surname.

To resolve this issue, please follow my steps below:

Firstly, make sure that this client’s first name, last name, display name
and alias are correctly reconfigured. Please see:

1. Expand to Server Management Advanced ManagementActive Directory Users
and Computers My BusinessUsersSBSUsers.

2. On the right pane, right click the user that you want to change the
surname, and go to Properties.

3. On the General tab, change the clients’ Last name (Surname), display
name, and the email name.

4. On the Account tab, you also need to change the User logon name.

5. On E-mail Addresses tab, check whether this client’s email name has been
changed, if not, highlight his/her email address, click Edit, and change
the email title. Click OK.

6. On Exchange Advanced tab, change client’s display name.

7. On Exchange General tab, change the Alias.

8. Click OK.

Secondly, you need to restart the Microsoft Exchange Information Store
service:

1. On the server, start -> run “services.msc” (without quotation marks).

2. High light Microsoft Exchange Information Store Service, right click it
and select Stop.

3. After that, right click it and select Start.

4. Go back to Exchange System ManagerServersYourserverFir­st Storage
GroupMailbox, right click on the right pane and select Refresh.

5. Test this issue again and see whether it works now.