When you send certified mail through USPS the certified mail will require a signature by the person getting the certified mail delivered to them.
However I've had a trusted neighbor sign for my certified mail before but some USPS delivery people will only turn the certified mail over to the actual person listed on the certified mail.
So if you send a certified mail to someone the USPS will get the persons signature and you'll have proof of delivery where it was delivered.
Some certified mail can be delivered to the address without the actual person being there to sign for it and someone 18 or over can sign for it and others will require the person listed as the addressee to sign for the certified mail.
But all certified mail requires a signature before it will be left with someone.