Make Outlook the default program for email, contacts, and calendar
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.