On 07/28/2011 06:23 PM, John R Pierce wrote: > Its the "References:" header that controls threading in mail clients > that support it. I'm using thunderbird, and afaik, it won't revert to > Subject based pseudo threading in the absence of References. Subject > line changes don't break the thread, which is why using 'reply' and > changing the subject to start a new thread makes a mess wherre the new > thread is buried in the original that it was in reply to.... there is a mix of what all are used, its mostly the in-reply-to that is supposed to be used, akait. References is also there, but introduced much later ( was it down to a Microsoft feature that they insisted get into the rfc ? ). See what your mail.strict_threading and mail.thread_without_re are set to; if you disable threading by subject, a very very large number of emails will lose thread sanity on this list. setting reply and changing subject does not change threads if your mail client is already doing the right thing. My argument here is that not all mail clients do, therefore sticking with subject line sanity would help increase the number of threads that can stay together. - KB