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GMail has changed the way in which emails are threaded in "conversation view" [1] and now the "References" header must also be set on the first mail in a thread. [1] https://gsuiteupdates.googleblog.com/2019/03/threading-changes-in-gmail-conversation-view.html Bug: Issue 10943 Change-Id: I146a0495994c35697c8dcc97bf8385cca9507510