Your Mac’s Mail app supports multiple accounts with separate inboxes, but you can’t add new accounts directly from the app’s settings. To change accounts, you’ll need to use the System Preferences.
How to Add or Remove Email Accounts in Mail
The accounts that Mail uses come directly from the “Internet Accounts” section in the system preferences. If you want to add a new account, you’ll have to do it there.
To launch System Preferences, click the Apple menu at the top of your screen and select “System Preferences.” You can also launch it from your Dock or your Applications folder.
Click “Internet Accounts” in the System Preferences window.
You’ll see a list that probably includes your iCloud account. To add a new email, click the “+” button at the bottom of the list, and then click on whichever mail provider you’re using. If you don’t see your provider, you can add your email manually with “Add Other Account.”
You’ll be shown a popup asking you to sign in to your mail account. Enter your email and password for the account you want to use, and click “Next.”
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Source : How to Add or Remove Email Accounts in Mail for Mac
