First, open the Mail app on your Mac.
Next, click "Mail" in the upper left-hand corner of the Menu Bar.
Select "Add Account" from the drop down.
Choose the Mail account type. In my case, mine is Google.
Enter in your email address and click "Next."
Enter in your password and click "Next."
Complete Two-Step Verification. You may be asked for a code that is sent to your phone number or other various two-step verification methods. Follow the on-screen steps and move to the next screen.
Click "Allow" to allow your Mac access to your Mail account.
Click the boxes next to the apps you would like to have access to your Mail account and click "Done."
Now, it's time for a little patience. Allow your mail to download. This could take minutes or hours, depending on how much mail you have.
If you've recently added a new email account to the Mail app on your Mac, then you know that your email will usually appear in the sidebar as is.