Mac Mail can be configured to send and receive email from all your other email accounts in addition to your Mac Mail account. You might have a Gmail, Yahoo, and Outlook email account delivering your mail to those addresses to your Mac Mail app. When it comes time to reply to one of them, you most likely want to use the email address the sender used to contact you. Mac Mail makes it easy to send a message from. Just click in the From field of any new message and choose the email address you want for the email from the drop-down menu.
The Mail application that ships with macOS and OS X is solid, feature-rich and spam-eliminating software that is also an easy-to-use email client. Optimized to work on the Mac, the Mail app is trouble free and full featured. Launch the Mail client in OS X (it's inside the Applications folder) and then go to Mail > Preferences. Click on the General tab and look for 'Default email reader'. Select another mail client or browser from the drop down menu and then close the Preferences window.