Here's the log from Zimbra.log just minutes before the stats is stopped:
May 31 23:59:13 zimbra postfix/smtpd[11861]: warning: 118.68.159.134: hostname adsl-dynamic-pool-xxx.hcm.fpt.vn verification failed: Name or service not known
May 31 23:59:13 zimbra postfix/smtpd[11861]: connect from unknown[118.68.159.134]
May 31 23:59:15 zimbra postfix/smtpd[11861]: NOQUEUE: reject: RCPT from unknown[118.68.159.134]: 554 5.7.1 Service unavailable; Client host [118.68.159.134] blocked using zen.spamhaus.org;
http://www.spamhaus.org/query/bl?ip=118.68.159.134; from=<boomqi267@remni-avtovoz.ru> to=<biopolis3@crescendas.sg> proto=ESMTP helo=<dynamic-pool-xxx.hcm.fpt.vn>
May 31 23:59:21 zimbra postfix/smtpd[11861]: lost connection after DATA from unknown[118.68.159.134]
May 31 23:59:21 zimbra postfix/smtpd[11861]: disconnect from unknown[118.68.159.134]
May 31 23:59:49 zimbra postfix/smtpd[11861]: warning: 118.68.159.134: hostname adsl-dynamic-pool-xxx.hcm.fpt.vn verification failed: Name or service not known
May 31 23:59:49 zimbra postfix/smtpd[11861]: connect from unknown[118.68.159.134]
May 31 23:59:49 zimbra postfix/smtpd[11861]: NOQUEUE: reject: RCPT from unknown[118.68.159.134]: 554 5.7.1 Service unavailable; Client host [118.68.159.134] blocked using zen.spamhaus.org;
http://www.spamhaus.org/query/bl?ip=118.68.159.134; from=<crabbiestn941@tisasoft.ru> to=<biopolis3@crescendas.sg> proto=ESMTP helo=<dynamic-pool-xxx.hcm.fpt.vn>
May 31 23:59:50 zimbra postfix/smtpd[11861]: lost connection after DATA from unknown[118.68.159.134]
May 31 23:59:50 zimbra postfix/smtpd[11861]: disconnect from unknown[118.68.159.134]
Jun 1 00:00:02 zimbra zimbramon[17074]: 17074:info: 2011-06-01 00:00:01, QUEUE: 44 7
Jun 1 00:00:07 zimbra zmmailboxdmgr[18036]: status requested
Jun 1 00:00:07 zimbra zmmailboxdmgr[18036]: status OK
Jun 1 00:00:19 zimbra zimbramon[3603]: 3603:err: Service status change: zimbra.dc1.crescendas.com stats changed from running to stopped