Rejected Mail - How to Fix This?

[Gary A.]

I've been sending out a lot of direct mail to addressed from a list I purchased on ListSource. It has worked well. However, I get a lot of rejected mail (return to sender, etc). Before I send out the mail, I run the list through the NCOA. However, I still get a lot of rejected mail.

Does anyone know how I can reduce the amount of direct mail that gets rejected? 

Direct mail marketing VETERANS are encouraged to respond. Thanks.