I'm opening a new bug report here, since the old bpo issue tracker apparently no longer exists and the ticket haven't been migrated.

bpo has an issue with email sigs which are longer than 3-4 lines (don't remember the exact details). Instead of removing the sign when posting to an issue via email, it takes the complete sig and adds it to the message posted on bpo, e.g. https://bugs.python.org/issue28468#msg381816