The following steps will set Outlook as your default mail program.
Make it your default email client
On the Tools menu in Office Outlook, click Options, and then click the Other tab.
Under General, select the Make Outlook the default program for E-mail, Contacts, and Calendar check box.
Note Making Outlook the default program for e-mail, contacts, and calendar does not automatically add or configure e-mail accounts in your Outlook profile. If you want to send or read e-mail messages in Outlook, you must add and configure the e-mail account as well.