I have new problem with my zimbra 7 which running on ubuntu 8.04.

Firstly no problem with them, i can send email, attach file, more than it, i can integrate with alfresco.

After i restarting my computer, now i cann't to send email.When i try to send email error message happen like this on message box

HTML Code:
n 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:	com.zimbra.cs.mailbox.MailServiceException: try again: Unable to connect to the MTA ExceptionId:btpool0-3://meruvian.go.id/service/soap/SendMsgRequest:1301308129806:9e68b678e28b01ac Code:mail.TRY_AGAIN at com.zimbra.cs.mailbox.MailServiceException.TRY_AGAIN(MailServiceException.java:478) at com.zimbra.cs.mailbox.MailSender.sendMimeMessage(MailSender.java:628) at com.zimbra.cs.mailbox.MailSender.sendMimeMessage(MailSender.java:425) at com.zimbra.cs.mailbox.MailSender.sendMimeMessage(MailSender.java:385) at com.zimbra.cs.service.mail.SendMsg.doSendMessage(SendMsg.java:193) at com.zimbra.cs.service.mail.SendMsg.handle(SendMsg.java:154) at com.zimbra.soap.SoapEngine.dispatchRequest(SoapEngine.java:406) at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.java:281) at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.java:158) at com.zimbra.soap.SoapServlet.doWork(SoapServlet.java:294) at com.zimbra.soap.SoapServlet.doPost(SoapServlet.java:215) at javax.servlet.http.HttpServlet.service(HttpServlet.java:725) at com.zimbra.cs.servlet.ZimbraServlet.service(ZimbraServlet.java:208) at javax.servlet.http.HttpServlet.service(HttpServlet.java:814) at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511) at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166) at com.zimbra.cs.servlet.SetHeaderFilter.doFilter(SetHeaderFilter.java:79) at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) at org.mortbay.servlet.UserAgentFilter.doFilter(UserAgentFilter.java:81) at org.mortbay.servlet.GzipFilter.doFilter(GzipFilter.java:132) at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388) at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:218) at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182) at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765) at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:422) at org.mortbay.jetty.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:230) at org.mortbay.jetty.handler.HandlerCollection.handle(HandlerCollection.java:114) at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152) at org.mortbay.jetty.handler.rewrite.RewriteHandler.handle(RewriteHandler.java:230) at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152) at org.mortbay.jetty.handler.DebugHandler.handle(DebugHandler.java:77) at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152) at org.mortbay.jetty.Server.handle(Server.java:326) at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:543) at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:946) at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:756) at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:218) at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:405) at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:410) at org.mortbay.thread.BoundedThreadPool$PoolThread.run(BoundedThreadPool.java:451) Caused by: java.net.ConnectException: Connection refused at java.net.PlainSocketImpl.socketConnect(Native Method) at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:351) at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:213) at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:200) at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:366) at java.net.Socket.connect(Socket.java:529) at com.zimbra.cs.mailclient.MailConnection.newSocket(MailConnection.java:416) at com.zimbra.cs.mailclient.MailConnection.connect(MailConnection.java:74) at com.zimbra.cs.mailclient.smtp.SmtpTransport.protocolConnect(SmtpTransport.java:183) at javax.mail.Service.connect(Service.java:291) at javax.mail.Service.connect(Service.java:172) at javax.mail.Service.connect(Service.java:121) at com.zimbra.cs.mailbox.MailSender.sendMessageToHost(MailSender.java:894) at com.zimbra.cs.mailbox.MailSender.sendMessage(MailSender.java:831) at com.zimbra.cs.mailbox.MailSender.sendMimeMessage(MailSender.java:565) ... 39 more
i try to check zimbra log with commnad tail -f /var/log/zimbra.log
and i get it

HTML Code:
Mar 28 17:39:00 meruvian postfix/postqueue[1595]: fatal: Queue report unavailable - mail system is down
Mar 28 17:39:30 meruvian postfix/postqueue[1660]: fatal: Queue report unavailable - mail system is down
what's wrong with my zimbra configuration ?

help me