it looks like zimbra is becoming a real exchange replacement. Email messages in the Sent Items folder have the same PR_INTERNET_MESSAGE_ID property in an Exchange Server 2010 environment describes a situation where an email in the sent folder is send again with outlook and not delivered. Using Outlook 2010 with ZCO the 'cloned' messages get the same Message-Id, so zimbra (and e.g. cyrus imapd) will silentliy discard the message (zimbra's log line is "lmtp - Not delivering message with duplicate Message-ID").
As Outlook 2010 does not use the same Message-Id in the same situation with an IMAP Account I think this must be a ZCO bug. Can anybody confirm this? Does it work with later zimbra versions?