[VM] Threading using subject

[Uday Reddy]

I was somewhat dismayed to discover a few days ago that the default value of
`vm-thread-using-subject' has been `t'.  I had assumed that the default
value was `nil' and most people weren't using this feature.

The normal method for grouping messages into threads is using their
"In-Reply-To" and "References" headers.  In the early days of email, these
headers were not common.  The standards weren't very good and many mail
clients didn't bother to follow the standards that did exist.  I also recall
that VM's threading code wasn't up to mark in the early days.  So, we had to
resort to grouping messages by their subject.

By version 7.19, VM's threading code was pretty good and the Internet
standards also improved.  It is now very rare to get messages without proper
headers.  (Some exceptions do exist.  Typically, messages generated software
- web sites or mailing list managers contain bad headers.)

So, I am wondering if I should change the default value of
`vm-thread-using-subject' to nil.

The reason for doing this is that reference-based threads and subject-based
threads conflict.  Essentially, subject lines can change in the middle of
threads, which means that you can't tell the subject thread of a message
just by looking at its subject line.  This makes VM's threading code very
complicated.  I think the code for reference-based threads is essentially
perfect now.  But subject-based threading has quite a few dark corners, some
that I know about and others I don't.  So, if the majority of the users
don't use subject-based threads, then these problems won't affect them.

So, the question is, should we change the default value of
`vm-thread-using-subject' to nil?

Cheers,
Uday