You might be required to show proof that you mailed something for any number of reasons. The nature of what you’re mailing can determine what qualifies as proof, but it all begins with the U.S. Postal Service.
Certificate of Mailing
If you only need to prove that you forwarded something, such as a tax payment to your county or state or to the Internal Revenue Service, you can request a certificate of mailing from the U.S. Postal Service. However, the certificate only confirms that you indeed mailed what you said you mailed. If you want proof that the envelope or package got to its intended recipient, you’ll have to send the letter or package by registered or certified mail, return receipt requested, rather than first class mail. You can also request tracking services or signature confirmation.
Proof of Service
If you’re serving legal documents on another party, this typically requires that you must send them by certified mail. A certificate of mailing won’t do you any good because the court’s concern isn’t so much that you mailed the documents, but whether the other party actually received them and is aware of the lawsuit. If you send the documents by certified mail, the recipient must sign a green card, acknowledging service. This green card qualifies as proof of mailing when you complete a proof of service form, often available from the court, and attach it. Some states take it a step further and require that you specify restricted delivery when you serve a lawsuit by mail.