1) Don't put it on there and let the letter be forwarded...this is what I do. A few reasons why but mostly due to the fact that I already stamp on the outside of the letter some other insignia such as "I Want To Buy This House", "Don't Lose This House To Tax Sale!", etc... it just depends on the target market I'm direct mailing to.

2) Stamp on the letter, "Address Correction Requested" and then get the owners new address and change your mailing approach. This has its merit if it is someone you're targeting to with multiple mailings.

Thanks,
Scott