I was interning in Rackspace this summer. For a while we were working on an email notification sender and we wanted email clients to group messages related the same event into a single conversation. However, different "event" might have same message subject, and we wanted to suggest how messages should be grouped. So we looked into "In-Reply-To" (RFC-822) and "References"(RFC-2822) fields. We ended up implementing RFC-2822 since it obsoletes RFC-822 and we figured if we want our message grouping work on most email clients, the safest way was to use the up-to-date standard (2822).<p>Interesting fact is that, among three clients we tested, only mutt faithfully implemented the standard. It honestly grouped all messages referenced to the same ID into the same parent, despite subject or sending time. However, neither Gmail or Outlook respects the "References" field.<p>In Gmail, it seems subject of the message plus [one of <time of message sent> and <References>] are used for grouping. But it certainly doesn't exclusively rely on "References" since we got messages referenced to same parent message grouped into different conversations.<p>In Outlook, "References" field is ignored completely. It only relies on subject of messages. We got messages for different "event" from more than 10 days from each other, grouped into same conversation.