[bensummers]

From the menus: View / Organise by thread

[evilbit]

It sucks. It blindly groups messages by subject, regardless of whether they're actually related, e.g. for me it grouped all empty subject messages from a span of 6+ years that I've had this Gmail account (I never delete email).

[intranation]

Gmail does that too—it doesn't respect the Thread-ID header (Mail.app does).