It is possible in most circumstances to send a letter without a return address. One must populate the destination name and address within the Optical Character Reader (OCR) area on the envelope and apply adequate postage before sending it for mailing.
The return address is typically listed in the upper left corner of the envelope. However, one can usually mail a letter without filling in the return address on the letter. Some situations where a return address is required, according to USPS, include Priority Mail shipping, package services, mail with extra services, placing a company permit imprint or paying for postage with pre-canceled stamps.
The destination address must be within proper boundaries for the letter to be deliverable. According to USPS, the destination mailing name and address should be at least in the OCR area, which is: 1/2 inch away from the left edge of the envelope; at least 1/2 inch away from the right edge; the top of the OCR less than 2 3/4 inches up from the envelope's bottom edge; and the bottom of the OCR greater than 5/8 inch away from the bottom edge of the envelope.