The best converting real estate email subject line we've seen recently is...
"Price drops near you"
This works for both sellers and buyers because both of these prospective client types have vested interest to investigate further (gain for buyers, urgency for sellers, both powerful motivators).
Redfin uses this effectively, promoting their search feature that lists properties with recent price drops. It's always a good practice to learn from the industry leaders, and borrow marketing and growth hacking techniques from the well funded marketing teams of large brokerages.
The cheapest and easiest way to be in the loop is to sign up to major portals and brokerages like Redfin and Zillow as a consumer, as a prospective home buyer or seller, - using some personal email and account - and see how they go about marketing to you. They spend a fortune on data analytics for their marketing, why not borrow some of their ideas. Look out for copyright issues, but other then that, it's free education on best practices.