If someone sends you certified mail and you're not at home to receive it, the letter carrier leaves a notification of the attempted delivery. You'll take this form to the local post office to collect your item. The delivery note lists the unique tracking number for the mail piece. You can use this number to find out where the item was sent from, but you cannot see the sender's details until you sign for the letter.
TL;DR (Too Long; Didn't Read)
The post office will not tell you who sent a piece of certified mail. Otherwise, you might refuse to accept unpleasant mail such as jury duty notices, tax demands or a summons to appear in court.
Who Uses Certified Mail
When mail is sent as a certified delivery, you have to sign to receive the item. Your signature, or that of an authorized agent, is proof that you received the mail. Law firms and government agencies typically use certified mail when they need a legally recognized proof of delivery, for example, when sending court papers, tax audit notifications or important contracts. There are dozens of reasons why someone may send you certified mail, not all of them bad. The signature operates as a type of receipt, so people often use certified mail when paying rent or sending money in settlement of an invoice.
Look Up The Tracking Number
Each certified mail item has a unique tracking number so the location can be traced at every stage of the journey. Most U.S. Postal Service tracking numbers are 22 numbers long, nine of which comprise the mailer ID. This is the only identifier you'll get for the sender; USPS will not give you the sender's name until you have signed for your mail. If you navigate to USPS' "Track and Confirm" web page and enter the tracking number, you'll be able to see the ZIP code of the post office from which the letter was sent. This may give you clues about the seller's identity.
Collecting the Item
When you collect your item, hand over the form the letter carrier left for you, and the post office worker will bring out your mail. You are not permitted to receive or open the item until you have signed for it. However, USPS rules stipulate that certified mail must bear the return address of the sender in the upper left corner on the front of the envelope. You may be able to read the address and determine the sender's identity before you make a decision about whether to sign for the letter.
Refusing to Accept Certified Mail
You don't have to accept certified mail or collect the item from the post office. USPS will try to deliver the item three times after which it will return it to the sender marked "unclaimed." Take care when refusing certified mail, however, since it could have legal ramifications. For example, if you refuse a tax demand from the Internal Revenue Service, you might miss the deadline for appealing and wind up with collection agents knocking at your door. You can't stop legal proceedings by refusing to accept certified mail about them. Evictions and court cases will still proceed, and your refusal will be entered as evidence.