Fixing undelivered mail from my direct mail campaigns

[Armaan Premjee]

Hello Bigger pockets members,

I have been sending out a lot of direct mail. I have been receiving my data from the county's delinquent tax lists. I have been filtering this data to delete any addresses that don't make sense. I am still getting 10% of my mail as undeliverable. Any advise on how I can verify these addresses beforehand?