Step 1: In the bottom-left corner, select the circle with your initials > Settings.
Step 2: In the left-hand banner, select Mail > Smart Mail.
Step 3: If your email is a Gmail account, then select the Link Account to Gmail button. Otherwise, select the Link Account to non-google email providers.
Email provider specific guides:
Gmail - Guide Here
Outlook - Guide Here *this is only needed if you have two-factor authentification
Mac Mail - Guide Here
Step 4: Sign in to your email account following the prompts, then select Allow. You should see a message that your email was successfully linked and you can begin sending Smartmail!