So I have built a prototype and I want to validate the idea.
Basically it is a social media data mining tool for developers to get feedback for their apps.
We retrieve unstructured data from different sources, apply data mining to the data and then we present colorful dashboards full of rich analytic that help mobile developers find out what people are saying about their apps, which features they like or don't, etc.
So as you can see, my clients are app developers or mobile development companies. The question is:
I'm afraid such a campaign of mass sending will fall into email spam, which is not only ineffective but also easy to be illegal. http://en.wikipedia.org/wiki/Email_spam I would suggest you to hire a freelance assistant to collect the contact page URLs of the developers website, compose custom message and submit through the form, which is the generally allowed method.
The custom message could mention his specific app, a demo url on your site to show stats for his app, and any other specific information to help if possible.
You can start with small batch to see response and then scale.
Don't email random people, as that's spam and will never get you anywhere.
In an ideal world you would have had a few representative users on board from day one to validate your ideas as you built the prototype. All is not lost though, but try to get some candidates on board as soon as possible. You need to avoid the common problem of creating an elegant solution to a problem that nobody has (or people have, but won't pay to solve).
You may try contacting companies through the request forms on their website (search for company directories). Consider reaching people on social networks like LinkedIn. Make a short video demo or slideshow, prepare screenshots and links with examples. Describe the benefits they will get while using your tool. After sending the offer you may call them in several days in order to get the respond.