Debug message: error while proxying request to target server: HTTP/1.1 503 Service Unavailable

Exception:

com.zimbra.common.service.ServiceException: error while proxying request to target server: HTTP/1.1 503 Service Unavailable
ExceptionId:sync-mbox-kopas@zmicro.com:1373043826133:941c9a3c6812dfc1
Code:service.PROXY_ERROR Argurl, STR, "https://mail.zmicro.com:4430/service/soap/SyncRequest")
at com.zimbra.common.service.ServiceException.PROXY_E RROR(ServiceException.java:322)
at com.zimbra.common.soap.SoapHttpTransport.invoke(So apHttpTransport.java:247)
at com.zimbra.common.soap.SoapHttpTransport.invoke(So apHttpTransport.java:164)
at com.zimbra.common.soap.SoapTransport.invoke(SoapTr ansport.java:407)
at com.zimbra.common.soap.SoapTransport.invokeRaw(Soa pTransport.java:381)
at com.zimbra.soap.ProxyTarget.execute(ProxyTarget.ja va:135)
at com.zimbra.soap.DocumentHandler.proxyWithNotificat ion(DocumentHandler.java:538)
at com.zimbra.cs.mailbox.ZcsMailbox.sendRequestWithNo tification(ZcsMailbox.java:608)
at com.zimbra.cs.mailbox.DeltaSync.sync(DeltaSync.jav a:103)
at com.zimbra.cs.mailbox.DeltaSync.sync(DeltaSync.jav a:89)
at com.zimbra.cs.mailbox.MailboxSync.sync(MailboxSync .java:192)
at com.zimbra.cs.mailbox.ZcsMailbox.sync(ZcsMailbox.j ava:124)
at com.zimbra.cs.mailbox.ZcsMailbox.syncOnTimer(ZcsMa ilbox.java:105)
at com.zimbra.cs.mailbox.SyncMailbox$1.run(SyncMailbo x.java:279)
at java.util.TimerThread.mainLoop(Unknown Source)
at java.util.TimerThread.run(Unknown Source)

If issue persists please post description and debug information here.