We can safely turn to the Post Office in this matter.
Yes, the post office does need to be contacted too if they made the delivery.
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This poster can be enlarged — links here:
http://www.usps.com/news/2001/press/mailsecurity/security.htm
Larger:
http://www.usps.com/news/images/poster_large.jpg
Largest:
http://www.usps.com/news/_pdf/poster.pdf