One of the most peculiar things is happening to a client of ours:

About 30 email messages from and to certain chinese mail servers failed to deliver today, with the following errors:

Zimbra admin mailbox:

Errors

2007-07-16 23:50:39 deferred (lost connection with mail.shaidc.com[218.78.208.139] while sending message body)
2007-07-16 12:08:33 bounced (host mx04.263.net[124.200.240.28] said: 551 User not local; please try <forward-path> (in reply to RCPT TO command))
2007-07-16 10:33:57 deferred (lost connection with mail.shaidc.com[218.78.208.139] while sending RCPT TO)
2007-07-16 10:33:56 bounced (host mail.shaidc.com[218.78.208.139] said: 551 User not local; please try <forward-path> (in reply to RCPT TO command))
2007-07-16 08:59:40 deferred (lost connection with mx.vip.163.com[202.108.5.193] while sending end of data -- message may be sent more than once)
2007-07-16 23:50:33 deferred (lost connection with mail.longerfashion.com[218.75.110.132] while sending message body)
2007-07-16 08:44:45 deferred (lost connection with mx.mail.163.split.netease.com[220.181.12.62] while sending message body)
2007-07-16 23:50:34 deferred (lost connection with mail.shaidc.com[218.78.208.139] while sending end of data -- message may be sent more than once)
2007-07-16 23:50:37 deferred (lost connection with mx.vip.163.com[202.108.5.193] while performing the initial protocol handshake)
2007-07-16 23:50:37 deferred (lost connection with mx.mail.163.split.netease.com[220.181.12.60] while sending end of data -- message may be sent more than once)
2007-07-16 23:50:44 deferred (lost connection with mx.vip.163.com[202.108.5.190] while sending message body)
2007-07-16 20:05:32 bounced (host mail.shaidc.com[218.78.208.139] said: 551 User not local; please try <forward-path> (in reply to RCPT TO command))
2007-07-16 23:50:33 deferred (lost connection with mail.longerfashion.com[218.75.110.132] while sending end of data -- message may be sent more than once)
2007-07-16 11:39:36 bounced (host mail.shaidc.com[218.78.208.139] said: 551 User not local; please try <forward-path> (in reply to RCPT TO command))
2007-07-16 08:46:34 deferred (lost connection with mail.longerfashion.com[218.75.110.132] while sending MAIL FROM)
2007-07-16 08:29:36 deferred (lost connection with mail.shaidc.com[218.78.208.139] while sending end of data -- message may be sent more than once)
2007-07-16 08:35:32 deferred (lost connection with mx.vip.163.com[202.108.5.193] while sending end of data -- message may be sent more than once)
2007-07-16 23:50:38 deferred (lost connection with mx.mail.163.split.netease.com[220.181.12.74] while sending end of data -- message may be sent more than once)
2007-07-16 08:19:30 deferred (lost connection with mx.vip.163.com[202.108.5.193] while sending end of data -- message may be sent more than once)
2007-07-16 23:50:36 deferred (lost connection with mx.zjip.com[202.101.172.203] while sending end of data -- message may be sent more than once)
2007-07-16 23:50:43 deferred (lost connection with mx.mail.163.split.netease.com[220.181.12.72] while sending RCPT TO)
2007-07-16 23:50:33 deferred (lost connection with mx.mail.163.split.netease.com[220.181.12.71] while sending message body)
2007-07-16 23:50:39 deferred (lost connection with mx2.cnm.cn[59.151.23.106] while sending MAIL FROM)
2007-07-16 11:39:38 bounced (host mail.shaidc.com[218.78.208.139] said: 551 User not local; please try <forward-path> (in reply to RCPT TO command))
2007-07-16 23:50:41 deferred (lost connection with mx.vip.163.com[202.108.5.190] while sending message body)
2007-07-16 23:50:37 deferred (lost connection with mx.mail.163.split.netease.com[220.181.12.66] while sending message body)
2007-07-16 11:39:32 bounced (host mail.shaidc.com[218.78.208.139] said: 551 User not local; please try <forward-path> (in reply to RCPT TO command))
2007-07-16 23:50:43 deferred (lost connection with mail.longerfashion.com[218.75.110.132] while sending message body)
2007-07-16 23:50:41 deferred (lost connection with mx.vip.163.com[202.108.5.190] while sending DATA command)
2007-07-16 23:50:47 deferred (lost connection with mx.vip.163.com[202.108.5.193] while performing the initial protocol handshake)
2007-07-16 01:30:02 bounced (fast2.info)
2007-07-16 01:35:27 bounced (fast2.info)
2007-07-16 01:35:59 bounced (fast2.info)
2007-07-16 01:54:59 bounced (fast2.info)
2007-07-16 23:50:44 deferred (lost connection with mx.mail.163.split.netease.com[220.181.12.67] while sending message body)
2007-07-16 11:53:36 bounced (host mail.shaidc.com[218.78.208.139] said: 551 User not local; please try <forward-path> (in reply to RCPT TO command))
2007-07-16 11:26:14 deferred (lost connection with mail.shaidc.com[218.78.208.139] while sending end of data -- message may be sent more than once)
2007-07-16 23:50:40 deferred (lost connection with mx2.cnm.cn[59.151.23.106] while sending MAIL FROM)
2007-07-16 23:50:42 deferred (lost connection with mail.shaidc.com[218.78.208.139] while sending end of data -- message may be sent more than once)
2007-07-16 18:16:40 deferred (lost connection with mx.vip.163.com[202.108.5.193] while performing the initial protocol handshake)
2007-07-16 23:22:28 bounced (host linuxserver.linuxnet.tx[192.168.0.77] said: 550 5.1.1 No such user here (in reply to RCPT TO command))
2007-07-16 20:01:31 deferred (host g.mx.mail.yahoo.com[206.190.53.191] said: 451 Message temporarily deferred - [190] (in reply to end of DATA command))
2007-07-16 13:33:37 bounced (host gmail-smtp-in.l.google.com[64.233.183.27] said: 552 5.7.0 Illegal Attachment f4si32391278nfh (in reply to end of DATA command))
2007-07-16 22:17:42 deferred (host g.mx.mail.yahoo.com[209.191.88.239] said: 451 Message temporarily deferred - [170] (in reply to end of DATA command))
2007-07-16 16:47:13 deferred (lost connection with mx.vip.163.com[202.108.5.189] while sending DATA command)
2007-07-16 11:56:12 deferred (host g.mx.mail.yahoo.com[206.190.53.191] said: 451 Message temporarily deferred - [190] (in reply to end of DATA command))
Zimbra logs (snip):

Jul 17 01:56:43 linuxserver postfix/smtp[3701]: EE0165691A4: to=<xxxx@163.com>, relay=mx.mail.163.split.netease.com[220.181.12.59], delay=7559, status=deferred (lost connection with mx.mail.163.split.netease.com[220.181.12.59] while sending message body)
Jul 17 01:56:40 linuxserver postfix/smtp[3704]: BD47B5691A5: to=<xxxx@longerfashion.com>, relay=mail.longerfashion.com[218.75.110.132], delay=7563, status=deferred (lost connection with mail.longerfashion.com[218.75.110.132] while sending end of data -- message may be sent more than once)
Jul 17 01:56:37 linuxserver postfix/smtp[3712]: 987AB5691A0: to=<xxxx@vip.163.com>, relay=mx.vip.163.com[202.108.5.190], delay=7556, status=deferred (lost connection with mx.vip.163.com[202.108.5.190] while performing the initial protocol handshake)
All of these are chinese domains, meanwhile mail with attachments delivers regularly in Italy and Europe.
I have tried pinging port 25 on these domains, and they all respond;
Googled about it, found something about MTU path setting in the kernel, but it seems to be regular.

DOES anybody have ANY idea about this?? :

I will appreciate anyone that could give a hint, clients are upset about not beeing able to send to their chinese partners...

P.S. I already tried to contact the administrators of such domains (through whois database), but haven't received an answer yet.