Mail is usually returned because it is missing either forwarding information or secondary information. You should always validate a mailing address before you mail something.
If you have returned mail pieces with both the address and the yellow nondelivery sticker affixed, you can find out why it wasn't delivered. The sticker has a code on it. You can find the official definitions of all the messages, but we've included the common ones below for convenience.
|Not Deliverable as Addressed—Unable to Forward||Mail undeliverable at address given; no change-of-address order on file; forwarding order expired.||Somebody moved but didn't tell the USPS to forward their mail or the forwarding period ended.|
|Attempted—Not Known||Delivery attempted, addressee not known at place of address.||This means the letter, as it was addressed, was delivered (or attempted) but then put back into the mail stream because the recipient didn't match the one currently at the address.|
|Vacant||House, apartment, office, or building not occupied.||Addresses marked as "Vacant" reached the destination terminal but were then returned as undeliverable because the mail carrier marked the address as vacant. The Postal Service can sometimes take up to 90 days to get a vacant status into their system. After that, the updates go out to us monthly and we implement them immediately.|
|Insufficient Address||Mail without number, street, box number, route number, or geographical section of city or city and state omitted and correct address not known.||The address is just plain bad. You should verify your addresses!|