Hello. We're running Release 8.0.0.GA.5434 FOSS edition on Ubuntu 12 x64.
We're receiving the following error when trying to send (or reply to) a message from the Zimbra web client:

'An unknown error (mail.TRY_AGAIN) has occurred'
method: [unknown]
msg: try again: Unable to connect to the MTA
code: mail.TRY_AGAIN
detail: soap:Receiver
trace: qtp162281097-385:
request: Body: {............... [I omit this part]

Clients receive and send mail using Outlook (using the Zimbra server as a SMTP server) and have no problem.
The problem occurs ONLY when using the zimbra web interface. We now need to use the Zimbra web client.
The zimbra.log file doesn't log anything about the messages sent by the Zimbra web interface
I have no idea how to fix this problem....Ideas?