Bad mailing lists are evil
It is estimated by the USPS that US companies lose approximately $611 billion annually on undeliverable-as-addressed (UAA) mail. UAA mail comes directly from the mailing list used.
UAA mail takes resources to mail it and to get it delivered; then it ends up being recycled or trashed.
The USPS estimates that up to 7% of addresses could be outdated because of moving; which is why every mailing the list should be NCOA processsed.
Steps you can take to reduce UAA mail include: NCOA (national change of address) registry processing, Zip+4 processing, ACS (address change service) processing, and AEC (address element correction) processing. These tools help clean your list up and reduce UAA.