It is a good idea to stay in touch with our sphere of influence.
Now if it is not related to Real Estate, it's even better - that's what even I believe.
Happy Grasshopper just does that - sends branded emails to your sphere of 500 or 1000 members every 3 weeks.
It worked well initially for me - and yes, many of my clients opened emails initially and responded, too.
However, it did not generate any lead for me.
So it works, if opening email is your goal. Generating referral by these emails, I doubt.