It's vital for mail addresses to be clear and properly placed in order for the United States Postal Service to be able to deliver mail appropriately.
Mail must have return and mailing addresses so that the Post Office knows where the mail is being sent and where it should be returned if necessary.
The return address should be placed in the upper left-hand corner of the envelope and should include a valid street address, city, state and ZIP code.
The mailing address should be placed near the middle of the envelope, on the same side of the envelope that the postage stamp is on. It should include a valid street address, city, state and ZIP code, as well as a recipient’s name.
Although there is not an exact location where the mailing address should be placed, it should be written or typed on a clear part of the envelope. If a typed label is used, the label should also be placed in a clear, open area.
The United States Postal Service website offers a very detailed manual on domestic mailing; see the link below for more information. In addition, Microsoft Word incorporates a label and envelope formatting tool that assists with address placement. It can even print the address directly onto the envelope itself.