A response to a job rejection email has a more lax feel than the formality of the job application process, but it still requires attention to a particular structure: Say thank you
Thought we regret to inform you that we cannot accept your proposal this time
If you don't receive the response you're looking for via email, don't bite back, be snide, or act unprofessionally when you respond
Express gratitude for your opportunity to interview
The reason behind this is that your proposal was 25% above our intended budget