Putting together direct mail list. Questions

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I've put together a list of distressed properties in my market and have conducted research through the county tax assessors website on these properties. In order to make my list the most cost effective once I begin sending mail I was wondering if anyone could clear up some of the things that I have seen in the tax assessors records.

1. Properties listed as a life estate or either joint trust- Are these properties worth mailing to and has anyone completed a deal with one of these types of owners. 

2. Properties with the owners address listed as a Po box #.- Why would an owner have this?

Any other tips on creating a more efficient direct mail campaign is greatly appreciated.