How to sort mail.app conversations by subject rather than sender?

Most of my work-related mail goes through a ticketing system. Whenever a customer or me or any of my colleagues sends a mail or replies to a mail, the ticketing system sends a copy of said message to my inbox.

When 'threading' or 'sort on conversation' was on in Snow Leopard's Mail.app, it used to group all mail with the same subject. This way I could easily spot when a ticket was not yet replied to be my or one of my colleagues.

Lion's mail.app seems to behave differently and seems to sort mail by Sender rather than Subject. Does anyone know of a way to make Lion sort conversations by subject rather than sender?

Right now it's very annoying:

You would expect all mails with subject test to be grouped together, but they aren't. I would expect that it respects the "In-Reply-To" header.

These are the relevant headers of the first mail with subject [#VZB-74211-323] test:

Message-Id: <[email protected]>

Second mail:

Message-Id: <[email protected]>
In-Reply-To: <[email protected]>

third:

Message-Id: <[email protected]>
In-Reply-To: <[email protected]>

fourth:

Message-Id: <[email protected]>
In-Reply-To: <[email protected]>

Surely there MUST be a hack somewhere or some plist to edit to fix this?

Ah! I think I have actually found the answer. In the Mail.app with Lion, it sorts by whether or not there is a Re: in the subject. So all of those ticket emails up there, which start with the ticket tag, would thread correctly if they started with Re: [#tag] instead. I am about to add Re: to my RT subject lines, to fix this problem for my own ticket traffic.

This works fine for me in Mail.app (even with Organize by Conversation checked):

View > Sort By > Subject

and while viewing messages you can switch sorting like shown: