We should expect a certain percentage will be returned. While the addresses are kept up to date fairly well, the reality is people do relocate. Having 5-10% of letters returned would not be unheard of. If it gets dramatically higher than this, people should let me know.
What I've been able to do for some is hand pick addresses as replacements. For example, if someone had received a letter returned for a "Stacy Wright" and had addressed the letter as "Dear Stacy", I could then send a "Stacy Wilson" address as a replacement.
It will be hard for me to do this for one-off requests, but if people have a handful I'm willing to help.