While it is customary for one to put a return address when sending a letter, it is not required. However, the U.S. Postal Service encourages people to include a return address when sending mail.
The U.S. Postal Service does not require including a return address when mailing a letter, according to USPS.com. However, the majority of people choose to write a return address so that the recipient knows who sent the letter or package. The U.S. Postal Service also encourages using a return address to allow for the return of a package that cannot be delivered, has a wrong address or encounters other problems that prevent delivery.