Email Macro Not working right.

I have the following Macro in a file that is shared through our Network. (Executes w/a button). It works great most of the time, however, the boss can't get it to work. We use Outlook 2000 for our In-office & outside email. But, when he clicks on the button, the email program that comes up is Outlook express & won't send to anyone inside the office. How can I specify the Macro to use Outlook only?

Sub SendSheet()

'Sends a copy of single Worsheet as email
Application.ScreenUpdating = False
Sheet1.Copy
Application.Dialogs(xlDialogSendMail).Show
ActiveWorkbook.Close SaveChanges:=False
End Sub