Frontier Car Group interview questions

I've applied for the position and went successfully through the first interview. After that, I was given a very demanding assignment to complete in just 3 days over the weekend. They asked me to design and code 3 different email templates. They specifically asked me to submit the source code and all associated files with it. This took me over 14h to complete. Even though I emailed them for feedback after a week, I never received a reply. Neither was I formally rejected. It is quite likely that this company exploits job openings to get work done for free. Beware!