Depends on whether you like to access email with a browser or prefer an email client app (e.g., Thunderbird, Outlook, mobile email client apps, etc.).
For the browser go to start.att.net and click the mail icon in the upper right.
For email clients choose your favorite email client and set it up to use the servers specified here and use a secure mail key for the account password. Read this (which includes a link to how to create a secure mail key).
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