There are very few businesses where you can make a list of names, phone numbers, email addresses of all your potential customers, but you can in real estate photography since real estate agents are listed on their company website along with their listings.
I recommend that every beginning real estate photographer make a spreadsheet that lists all the listing agents in all the major real estate offices in their area. Here's how do that:
The most successful listing agents have the most listings at the highest price. Ideally you want to focus on the top 5% or 10% of agents. This may sound tedious but trust me this exercise will be the best time you spend marketing because it allows you to focus your marketing efforts on your most likely clients. Depending on the size of the area you are in this exercise could take a few hours or a few days, but it's well worth the effort.
As a real estate photographer you need to know the names of all the top listing agents in your area. During this process you can easily check to see if an agent with lots of listings are already using a professional photographer. This will tell you exactly who to focus your marketing efforts on by name.
How you use this list will depend on how much competition there is in your market area. In a market like Seattle, WA where there is a lot of real estate photography competition as a beginning real estate photographer you'll have to compete with a lot of folks. See what they are delivering and find a listing agent to turn into a satisfied customer. If you are working in Corvallis, OR where there are no real estate photographers you'll spend most of your time convincing listing agents that using a professional photographer will make them money. Either way success comes from making personal contact with those agents that are on your list of potential customers and creating a satisfied customer one at a time. Pretty soon the word about you will get around.