RFC 6068, section 6.1 has the following example:
An interesting use of 'mailto' URIs occurs when browsing archives of messages. A link can be provided that allows replying to a message and conserving threading information. This is done by adding an In-Reply-To header field containing the Message-ID of the message where the link is added, for example:
<mailto:list@example.org?In-Reply-To=%3C3469A91.D10AF4C@ example.com%3E>
In this example, a mailto link is used to reply to an existing email, using the URL encoded Message-ID. Despite this, I am so far unsuccessful in making this work in google chrome, neither the windows 10 mail client, nor the gmail service handler appears to respect this. The message ID's I have used were in the respective inboxes for both of these clients.
Is this feature actually supported by any mail clients or browsers?