If you’re like most people, you have more than one email program on your computer. You can simplify your life by making Outlook a one-stop center for sending and receiving email, storing your contacts, and scheduling appointments.
Click File > Options > General.
Under Start up options, check Make Outlook the default program for E-mail, Contacts, and Calendar.
Tip: Before you can send email messages, you need to add an email account to Outlook.