I am noticing something odd with the Zimbra client. When replying from a traditional email client, such as Mac OSX Mail.app or Thunderbird, the In-reply-to header is attached and honored; however, it does not appear this is the case with the Zimbra (web) client.
Is this considered the norm for the ZCS client?
I saw this old bug/RFE (early 2005) that leads me to think this might be a bug in 3.1.1 - http://bugzilla.zimbra.com/show_bug.cgi?id=375. Or quite possibly, something wrong with our setup.