There is no law that states one has to sign?

You don't have any right to someone's personal address if they don't want to make that information public. If they own their home in the name of an entity and use a PO Box for mailings, you cannot get their home address.

Are you trying to serve someone with documentation or confirm that something was sent? That certified letter, which is properly addressed and was certified, will do the job. As long as it is the most recent contact information available, you should be fine.

What is the purpose of your certified? I can help if you re-post.

No law requires someone to sign. But if they don't sign, they don't get the piece of mail.

Search local property records for the person's name. You can make sure they get the piece of mail by hiring a process server or getting delivery confirmation through the USPS.

If they are refusing mail from you, and you need to mail some sort of legal paperwork to them, you consult with an attorney on alternate ways to deliver the documentation. Typically, delivery confirmation will suffice if the person is refusing your signature required mail.

If you need a home address, you hire someone to find it.

If you need proof of delivery on a certified letter, you get the optional signature card service at the time of mailing. The recipient has to sign to get the letter and the signed signature card comes back to you.