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.”
Once you’re signed in, you’ll have the option to choose which features to link to your email. If you just want Mail, disable the other features like Contacts and Calendar, and click “Done.”
After that’s done, you will see a new account in the list. If you open up the settings for the Mail app (Command+Comma) and click on the “Accounts” tab, you’ll see your account linked in Mail. You can change the settings for that account from here.
If you want to remove a mail account, you can either turn off “Enable this account” in the Mail app’s settings, or remove the account from the “Internet Accounts” panel in System Preferences. Click the account, and then click the “-” button at the bottom of the list to remove it from your list of accounts.