In email terms, a bounceback is a category of error that in this case meant that the email address didn't exist. Although, that's because I emailed opt-out instead of arbitration-opt-out. My mistake. After fixing it, it went through without a reply as has been the general experience.
I'm glad I'm not the only one. I'm not using the application until I get a response. For all I know, they may respond on the very last day or not respond whatsoever.