The message line that was automatically sent says "This message was automatically generated by Gmail". It means that you marked a previous received message as spam, so it automatically sends a notification to Google in order to block further reception or messages from the same sender. Nothing to really worry about.

How do I make an auto generated email?

Automatic Filter and Forward

Some email platforms, such as Gmail, have email filters you can employ to generate automatic forwards of specific incoming mail to other email accounts. In Gmail, for example, you click on the "gear" icon at the top right of your Gmail page. Select "Settings" from the drop-down menu.

What does automatic generated mean?

Automatically generated (also called "auto-generated"—content) is content that's been generated programmatically. In cases where it's intended to manipulate search rankings and not help users, Google may take actions on such content.