I am noticing that every so often, when I reply to a email message, the recipient does not receive it. Now I am pressing reply to a message they just sent. I can roll-over the name and see their address book card... but when I send the message... they never receive it. Now it does get saved in my Sent items and it is not in any of the mail queues from the admin console.
Now I know that it is Zimbra, because we have an outbound mail gateway and there is no log of these messages ever being received by it.
Unfortunately I can not report it as a bug, because I can not put my finger on any specifics. From the user gui point of view, the message sent fine.