Iam using zimbra 6.0 opensource version.The server was working fine for the last 3 months.But last week i noticed so many emails are waiting in the mail queue.Even after i requeued the mails its error message is as follows

Jan 13 21:29:42 pop postfix/smtp[27275]: 0E72013C8002: to=<axxxxxx@gmail.com>, relay=127.0.0.1[127.0.0.1]:10024, delay=738, delays=730/0/0/7.4, dsn=2.0.0, status=sent (250 2.0.0 Ok, id=07465-18, from MTA([127.0.0.1]:10025): 250 2.0.0 Ok: queued as 67C9013C8030)
Jan 13 21:29:42 pop postfix/qmgr[8857]: 0E72013C8002: removed


Other error message

Jan 13 21:31:36 pop postfix/smtp[12311]: B1C6413C800C: to=<r.xxxx@xxx.net>, relay=mx-03.xxxx.net[146.145.49.174]:25, delay=900, delays=0.01/0.01/900/0, dsn=4.4.2, status=deferred (conversation with mx-03.xxxxx.net[146.145.49.174] timed out while receiving the initial server greeting)
Jan 13 21:31:36 pop postfix/smtp[12311]: B1C6413C800C: to=<txxxxam@xxxxxx.net>, relay=mx-03.xxxxxx.net[146.145.49.174]:25, delay=900, delays=0.01/0.01/900/0, dsn=4.4.2, status=deferred (conversation with mx-03.xxxxx.net[146.145.49.174] timed out while receiving the initial server greeting)
Jan 13 21:31:51 pop zmmailboxdmgr[32179]: status requested


I cant send any message to outside.Pls help me to settle this problem.
Thanks in advance