On Tue, May 05, 2020 at 11:02:46PM +0200, Andreas Schwab wrote:
> On Mai 05 2020, Philippe Vaucher wrote:
> 
> > I noticed that my Gmail threads started to be all split up.
> >
> > I traced this to some clients change the subject line to "RE: foo"
> > instead of "Re: foo" like this other clients.
> 
> Threads are defined by In-Reply-To and References headers, not by
> Subject.

Sometimes. Ideally. Some MUAs fudge it whenever the above are missing.
Some MUAs totally b0rk it :-)

Cheers
-- t