Robert Mast <beheerder@tekenbeetziekten.nl> writes:

> I ran git send-email and became the following line in the mail-header:
>
> "X-Mailer: git-send-email 1.7.9.5"
>
> Can I assume, apart from the version number, that this header-marker applies
> to all git-mail that should not be subject-splitted?

Hardcoding particular headers sounds too fragile to me. With that said,
if you want a corpus of email to investigate, there is e.g. 

http://notmuchmail.org/corpus/

Unfortunately I seem to recall threading is mostly pretty trivial,
except in the notmuch mailing list archive. If you prefer a smaller
download, that is at 

          http://notmuchmail.org/corpus/

(as an mbox)