Unknown automatically generated email in sent folder

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.

It's most probably an automatic email sent by Gmail but based on your explicit operation to choose "Unsubscribe" within Gmail.

Gmail provides a simple way to unsubscribe some mailing list taken by bots or added by spammers. So you can click "report spam" and "also unsubscribe" to get removed from those mailing list services.

Clicking "Unsubscribe" will automatically send a request back ( send an actual email from your Gmail account ) to the sender so they'll stop emailing you."