You've posted this in the Zimbra Desktop forum, is that where the problem is or is it a Zimbra server? You also haven't given any details about which version of Zimbra is installed. Update your forum profile with the output of the following command (do not post the output in this thread):
Some details of what you've already tried for debugging this problem might be a good starting point and whether this is a new install or an upgrade of Zimbra and whether the problem has just started happening. Have a look through the
log files and see what errors might be related to this problem.