I'm embroiled, on another thread and a bug, in "Zimbra webclient doesn't properly thread messages by In-Reply-To".
(And, no, that's not a feature request, it is a bug. It's a bug that Gmail doesn't do it, as well.)
Anyway, the followup to that is, "Does the Z webclient properly handle messages with RFC 2369 mailing list headers?"
Specifically, mailing list traffic should prompt a Mail User Agent, when asked to reply, to do one of two things:
1) Prompt the user whether to reply to the list or the sender, or
2) Reply to the list, unless the recipient uses a separate command to reply to the sender,
where the definition of "the list" is "the value of the List-Post header in the message".
(Some lists don't have the 2369 headers -- though all Mailman lists do -- the preferred approach here, due originally to Mutt, is to be able to add the list's To: address to a list, define it as a mailing list, and have the MUA behave this same way, generating replies with the To: address set to the *To:* address of the message.)
Something tells me that if we don't handle In-Reply-To right, we're not going to get this one right either...
Anyone handle mailing list traffic on Z? How's that working out for you?