How do I change the e-mail program that Office 20113 uses? Say I have an Excel spreadsheet open that I want to share..... I go to File : Share : E-mail : Send as Attachment. I want Excel to use Outlook. It's currently using Thunderbird. Windows 10 machine. In Windows Settings, Outlook is set as the Default e-mail program. In Outlook, the box beside "Make Outlook the default program for E-mail, Contacts, and Calendar" is checked. As far as I can tell, this should make the Office 2013 applications use Outlook. But they still open Thunderbird and start an e-mail using the first/default Thunderbird account.