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Keep in mind all steps below are safe and reversible. As long as you don't purposely delete your current the Mail folder inside of your "My Documents" folder (x:\mail folder ( or d:\mail on laptops), you should not have to worry about losing any old mail. Once thunderbird has converted your mail and address books correctly, we can go ahead and delete the old mail folder at a later date. As in the past, this Thunderbird will be setup set up to only view your UCI e-mail only. (No personal e-mail accounts are supported)
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