Windows 8.1 has a very capable modern UI mail program simply called Mail. Installing Outlook 2013 does not fully replace it because one default remains with Mail, the mailto association. To change the association to Outlook, go to Control Panel, select Default Programs, and then select Set your default programs.
Figure 1. Default Programs. Select Set your default program.
Figure 2. Notice that not all of the defaults are associated with Outlook 2013.
Figure 3. Notice that mailto is not associated with Outlook 2013.
Figure 4. Select Set this program as default to make Outlook 2013 the default for all mail related actions.