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Dear JACUSTOMER - There is no law that you must pick up certified mail. If this is a subpoena they can try to serve you personally if you refuse to pick it up. Obviously you will not know what is there if you don't pick it up so I can't offer any legal help for something that I have no knowledge of.
You can't get into trouble for not picking up your mail but that doesn't mean that whoever sent you whatever it is will not try to serve you personally.