Don't worry about it. They have acknowledged receipt of it. Just like sending something recorded delivery, you have no idea who actually signed for it or how it is handled after receipt.
You've fulfilled your obligation. The auto-reply you've received is a valid acknowledgment that Horizon Parking has received your communication. This is important because it provides a timestamped acknowledgment of receipt, confirming that your email has reached their system.
A delivery receipt or read receipt from an email can be used as evidence that the document was received, but it is not conclusive proof of service. Courts have ruled that service is valid once the document is delivered to the recipient’s server, regardless of whether the recipient actually reads the email.