E-mail Netiquette: How to Start a New Thread
You should not reply to an existing message in order to start a new thread, which will also involve changing the E-mail Subject:
line and deleting the existing quoted body. That's because standard-compliant threaded mail user agents (MUAs) will respect the In-Reply-To: headers and display the message as a reply to the one to which you replied.
The proper way to start a new thread is to:
Press the compose a new message button.
Fill in the mailing list’s E-mail address (you should have auto-completion and/or an address book).
Fill in the subject line and the body.
Licence
This document is Copyright by Shlomi Fish, 2013, and is available under the terms of the Creative Commons Attribution License (CC-by) 3.0 Unported (or at your option any later version of that licence).
For securing additional rights, please contact Shlomi Fish and see the explicit requirements that are being spelt from abiding by that licence.