Just some thoughts ... If the topic changes, starting a new thread is better than just changing the subject line, as I may not be interested in the topic that starts the thread, but if only the subject line changes and the thread doesn't change, my threaded e-mail client will usually delete messages I'm interested in along with the messages that didn't interest me at the beginning of the thread. It probably makes little difference to readers who don't use threaded mail clients, but those of us who do are greatly helped when a thread stays on topic as much as possible, and a new topic, even if it springs from a discussion on another thread, starts a new thread. That said, it can be hard to determine when a new thread should start when a new topic springs from a discussion on one thread, depending how related to the original topic the new one is, and how far it could stray in the future. In such cases, simply changing the subject line is probably sufficient. ~Kyle