[euank]

> Email has never had explicit threading

The "In-Reply-To" header is described in rfc2822. It is an explicit header in the RFC that is how you create threads.

Every mail client I've used correctly understands how to thread reply-chains using In-Reply-To.

The thing you're talking about, steam receipts grouping, is not a feature of email, but a specific feature of gmail's web view which is not mandated by any RFC and indeed is not explicit threading...

But there is a real way to thread which is defined in the RFC, and if you use a reasonable email client (aka not gmail), every mailing list's threading will work for you.