Gmail shows 'mailing list' information for messages I sent but I did not use a mailing list!
Gmail pulls the 'mailing list' information from the list_unsubscribe header that we add to all non-transactional messages. We do this as a means to provide all recipients the option of unsubscribing, which is a part of our policy and effort to adhere to good sending behavior as outlined by internet authorities. This header does not appear for transactional messages. For the difference between transactional and non-transactional messages, please see this article.
How do I remove this?
Unfortunately since the header is auto-generated it cannot be changed or removed. For more information about the list_unsubscribe header please see this article.