Issue with current broken build email notifications is that people not related to braking product build also get emails and naturally consider them as spam. This situation compromises email notifications being significant part of CI.
In my case product is combined of shared components and put together using subversion externals. In fact this subdivision can act as a hint narrowing commiter set responsible for breaking build.

Requirements:
Warnings plugin should return source file path which broke project build.

My suggestion:
Step 1. Get source file path that broke the build.
Step 2. Using subversion get svn:external under which broken source file is located.
Step 3. Get change commiters from previous stable build who commited to appropriate external from Step 2.
Step 4. Email notifications of broken builds to commiters from Step 3.

P.S. Other ideas on how to narrow notification receiver set would be interesting to read.