It doesn't even need to be personal. Just send them a notification notice. Nothing about this has to be personalized, it's already going to their email. Anything more and it's just bs. Even the notification of rejection email is already kinda bs because when you never hear back, you know you've been rejected, and the email itself is a functional courtesy. It could literally just be:

Email:

From: company@company.org

To: you

Subject: jobname application rejection date

Body: none