The USPS is constantly upgrading and can automatically flag marketing-related mail to be returned to sender.
This can happen easily if the recipient name doesn't actually map to a current known name/resident according to the postal service.
Thanks.io has addressed this by automatically appending 'Or Current Resident' to the recipient name to avoid this issue altogether.
Check out the short video above for more information.
Settings: Card Builder
You can set each address in your mailing list to be appended by using the checkbox in the Card Builder (right before placing your order for purchase.)
You can set each recipient in an automatic campaign to be appended with 'Or Current Resident' as well.