For several years now the United States Postal Service photographs every letter. The photograph and bar codes are most likely used for routing the letters. But knowing both the sender and receiver of correspondence allows inferences to be made. Additionally, this information is in plain view, there is no reasonable expectation of privacy and no warrant is required.
Based upon reliable infromation from a USPS employee it appears the system for photographing letters involves three steps.
First, only the front of the envelope is photograph and a light colored color bar code is sprayed on the back.
Second, the photograph is transmitted to a remote site where non-USPS employees at terminals input the letter’s bar code.
Third, a second machine reads the light colored bar code on the back and then sprays on a regular dark colored bar code on the lower front of the envelope. However, if the letter is bar coded, such as from a utility company, then it likely will not be bar coded again.
Fourth, while the duration for storage of the photograph is not known it is likely less than a week.
There are two solutions to avoid having a return address appear in the photograph.
First, include no return address. This has several problems because you may not know whether the letter is delivered, lost, etc. or the contents may be lost, fall into the wrong hands, etc. But it may be the best course of action if you are mailing extremely sensitive items or documents and do not want your address associated with the piece.
Second, place the return address on the back of the envelope. Because only the front of the envelope is photographed and because the postal worker sorting a return from insufficient postage, an error with the delivery address, lost, etc. letter will easily be able to find the address on the back of the envelope therefore this seems like the best action to take.
This also prevents your mail from going to one of the USPS’s three major mail recovery centers in Atlanta, San Francisco or Saint Paul. If your letter ends up in that purgatory then it will be opened and examined.
With the advent of technology the use of snail mail is becoming increasingly tracked. By putting the return address on the back of the envelope a key piece of information is not photographed but is still available should it be used. Additionally, using a ghost address and other techniques and tools found in the book How To Vanish will further enhance your privacy.