I guess you’re right and the presence of the List-Post header understandably makes Thunderbird think it’s a mailing list email. Probably a Reply-To header would suffice.
The following is from RFC 2369 - The Use of URLs as Meta-Syntax for Core Mail List Commands and their Transport through Message Header Fields
3.4. List-Post
The List-Post field describes the method for posting to the list. This is typically the address of the list, but MAY be a moderator, or potentially some other form of submission. For the special case of a list that does not allow posting (e.g., an announcements list), the List-Post field may contain the special value “NO”.
Examples:
List-Post: mailto:list@host.com
List-Post: mailto:moderator@host.com (Postings are Moderated)
List-Post: mailto:moderator@host.com?subject=list%20posting
List-Post: NO (posting not allowed on this list)