Open Mail for me
The first time you open Mail, you may be asked to add an account. Select an account type (such as Google for Gmail) or Other Mail Account, then enter your account information.
If you already added an email account, you can still add more. In Mail, choose Mail > Add Account, select an account type, then enter your account information. Make sure the Mail checkbox is selected for the account.
If you’re using an account on your Mac with other apps, such as Contacts or Messages, you can also use that account with Mail. In Mail, choose Mail > Accounts to open Internet Accounts settings, then click Add Account on the right.
You can View email account information in Mail on MacMail Support website