BKB, I would also consider the following when closing a CD via the mail (or any account, for that matter):
1. Verify the signature.
2. Verify the mailing address.
3. If either is out of sync with your records, I would go further, such as calling the customer to verify their identity (ask some personal questions).
If I still felt uncomfortable, I would asked for a notarized withdrawal slip and waive the penalty if time is an issue.
Hope this helps.
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Get your facts first, then you can distort them as you please. - Mark Twain