Threading not working properly is a result of some email clients not implementing the standard properly. RFC 2822 [0] has clear definition on threading using I

Threading not working properly is a result of some email clients not implementing the standard properly. RFC 2822 [0] has clear definition on threading using In-Reply-To and References fields. Its precedent RFC 822 [1] had definition on In-Reply-To field. However, not all email client implement this feature properly. As a result, there a lot of "conversational" messages flying around without proper In-Reply-To field. To solve this, Gmail builds threading trees not only based on In-Reply-To and References fields, but also heuristically based on the subject line of emails. Outlook, on the other hand, ignores In-Reply-To and References fields and groups messages into threads solely based on subject lines (according to last time I checked). And then, allowing (or requiring) emails to have similar subject lines to be properly threaded implicitly means it's OK for those client that don't produce proper In-Reply-To/References fields to continue doing so. Therefore, for any email client to properly thread all types of messages, it has to implement subject line based threading too. Things would be much easier if everybody had faithfully implemented the standard. [0] http://www.ietf.org/rfc/rfc2822.txt http://www.ietf.org/rfc/rfc2822.txt [1] http://www.ietf.org/rfc/rfc822.txt http://www.ietf.org/rfc/rfc822.txt EDIT: added RFC links