For these addresses not corrected, I would be able to provide a list with explanation(s) why the address was not deliverable. My business will be able to provide the necessary output for the customer in whatever form he desires. From the existing database, I can provide mailing labels, ...
How often is the list updated and/or cleaned (a "cleaned" list is one that has been processed by the U.S. Postal Service to flag recent changes of address and remove old non-deliverable addresses)? How frequently do people on the list make purchases, and how much do they spend?
Plain window envelopes won’t be the first thing addressees open when they retrieve their mail from the mailbox. Pieces that look more interesting will get the most attention. But that doesn’t mean you should abandon window envelopes — especially if you use windows to reveal something interes...