Your Mac's mail app supports multiple accounts with separate inboxes. However, you can not add new accounts directly from the app settings. To change accounts, you must use the system settings.
Adding or removing e-mail accounts in Mail
The accounts used by Mail come directly from the "Internet Accounts" section of System Preferences. If you want to add a new account, you have to do it there.
To start system settings, click the Apple menu at the top of the screen and choose System Preferences. You can also launch it from your computer dock or your application folder.
In the System Preferences window, click Internet Accounts.
You will see a list of your iCloud account. To add a new e-mail, click the "+" button at the bottom of the list and then click on the e-mail provider you are using. If your provider does not appear, you can manually add your email to "Add another account".
A pop-up will prompt you to sign in to your email account. Enter your email address and password for the account you want to use and click Next.
Once you're logged in, you can choose what features to use Link to your email If you only want email, turn off other features like Contacts and Calendar and click "Done".
. After that, a new account will be displayed in the list. When you open the Mail app settings (Command + Comma) and click the Accounts tab, your account will be linked to Mail. You can change the settings for this account here.
If you want to remove a mail account, you can either disable "Enable this account" in the Mail App settings or remove the account from the "Internet Accounts" section in the system settings , Click on the account and then on the "-" button at the bottom of the list to remove it from your account list.