If you have installed multiple e-mail programs on the same computer, the e-mail messages may not be sent from Outlook when you send email messages from any non email program.
To change the default mail client in Microsoft Internet Explorer, follow these steps, as appropriate for the version if Internet Explorer that you are running.
1. Start Internet Explorer
2. On the [Tools] menu, click [Internet Options].
3. Click the [Programs] tab
4. In the emai list, click Microsoft Office Outlook.
This process can also be done through the Control Panel > Internet Options > Programs > Set Programs
Verification Source: Microsoft Knowledge Base Article #154359