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Click on Tools -> Account Settings -> Account Actions -> Add Mail Account. If your menus on the top are hidden, click on the ALT key to show them, or access Account Settings by clicking on the radiator icon, on the top right of the interface, then Options -> Account Setting -> Account Actions -> Add Mail Account.
Enter the user's full name, e-mail address and password:
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Click on Continue. Thunderbird will automatically find the proper server names.
Click on Create Account