When using the RequesterRecipientProvider class, it can get stuck in an infinite loop if a build ends up being upstream of itself.  The root cause looks to be in the plugin that provides "Rebuild" as an option, but nonetheless this code should handle the case where the build's upstream build is itself to prevent an infinite loop.

https://github.com/jenkinsci/email-ext-plugin/blob/master/src/main/java/hudson/plugins/emailext/plugins/recipients/RequesterRecipientProvider.java#L55 

cur = p.getBuildByNumber(upc.getUpstreamBuild());

could be changed to the following to prevent an infinite loop

Run<?, ?> upstreamBuild = p.getBuildByNumber(upc.getUpstreamBuild());
if(cur == upstreamBuild) {
    context.getListener().getLogger().print("Somehow build " + cur + " is upstream of itself");
    break;
}