Just some thoughts off the top of my head. Aside from the Reg P considerations and any pre-screening (if involved), I think consistency is the key. If you decide to not send mail to everyone on the list, that decision should be backed by objective reasoning. Be careful with geography-based decisions as they could exclude a disproportionate amount of members of a protected class and be seen as disparate treatment or redlining depending on the demographics of those areas.
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I've just writed a wrong.