What are you doing to track down owners of your letters returned marked “ undeliverable” or “ insufficient address”?
Good to meet you Phillip
What to do with the non deliverable mail?
Lets first look at the NCOA system which is a service the USPS requires addresses to be run through prior to using a Barcode. The sytem takes an address and updates it with the "new" forwarding address of the person the mail is being sent to.
Which means the new address is the best last known address of the post office. So if you are getting mail back it is a sign of one of two things.
One- the person hasn't updated their address or two - the postal carrier is being lazy. I bet number 2 is happening more then number one as carriers tend to look at business mail as second class mail.
Occasionally you will get a piece marked Vacant or Deceased. Those are he only two types I would worry about.