It seems if you try and forward (and possibly reply) to certain HTML emails or emails with attachments from a different email address than the one that received the message it can cause the error "No such message exists"
The email in question was a HTML formatted email addressed to user2@mydomain where I wanted to forward it someone using my personal address.
user2@mydomain is a shared mailbox which I have access to as well as permissions to send as, by changing the from address in the compose window firstly HTML formatting seems to get lost, secondly zimbra may create attachments for any images that were embedded.
Then when trying to send the "No Such message exists" error is displayed.
I did hit the send error report button and I was going to file a bug But I'm not sure if I'm doing a good enough job of describing the problem in such a way the Devs would be able to try and repdroduce it.
Version 8.06GA, using Chrome on windows and OSX not tested other browsers yet