I am getting this with 5.0.9. User is sending the message via Outlook 2003 and is using the Zimbra Outlook connector.
Error: This message could not be sent.
Subject: FW: TA Send Message & PA Read Teacher Message
To: "user@mydomain.com" <user@mydomain.com>
Note: Soap Fault. Please recreate and resend the message. Details below:
Error Code: service.FAILURE
Error Text: system failure: MessagingException
<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope"><soap:Header><context xmlns="urn:zimbra"><change token="9788"/></context></soap:Header><soap:Body><soap:Fault><soap:Code><soa p:Value>soap:Receiver</soap:Value></soap:Code><soap:Reason><soap:Text>system failure: MessagingException</soap:Text></soap:Reason><soap

etail><Error xmlns="urn:zimbra"><Code>service.FAILURE</Code><Trace>btpool0-10:1222882708039:d6f7a6ebcb291667</Trace></Error></soap

etail></soap:Fault></soap:Body></soap:Envelope>