1,000 letters every other month- Who should I target?

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Hi everyone, 

I'm planning to send 7,000 letters (1,000 letters every other month or 45 days) to either absentee owners, probates, delinquent taxes etc. This will be a big expenditure for me and I want to make sure this campaign is properly executed. To which group do you think I should target? Where should I get the list from? And what criteria should I use for my mail campaign? 

I've used list source before, however, I haven't been very successful with it since my past campaigns were very inconsistent. I'm also not sure what would be the most effective criteria. Any feedback would be greatly appreciated.