Is it illegal to market to high equity and absent home owners?

[Dustin Erlewine]

I am considering getting my RE license.  I had my license for a short stint in 2008-2009.  I have been direct mail marketing to absent home owners and high equity home owners that I found on listsource. I am looking to purchase these homes for flips and rentals.  The response rate was 10%.  I had quite a few leads that just wanted me to list there house.  So I called a family friend as asked if he was interested in hooking up one of my leads.  During the conversation he explained that its illegal for RE agents to market this way and would have to send mailers to everyone on that street.  He explained it opens the chance of a law suit and losing your license.  I was surprised and this didn't make sense to me.  Well during a conversation with a different realtor in the same area I was told the exact same thing.  Which leads me to believe either they are correct or there's someone spreading bad information on this.   I was considering getting my RE license sense I am coming across so much free chicken with the listings.  However, I don't want to waste my time if I can't market the way I am now.  What is the right answer?  I am in Missouri.