Email bounce back Winsock Error 10054 Connection was reset by the other side!

Hi Experts,
I always got this problem when sending email to the domain as the trans log below. My email SMTP server is MDaemon 9.6.6 other server is CentOS 5.2 with Zimbra/Postfix.

Anyone know what exactly the problem and solution please help.

Thanks,

Wed 2009-06-03 15:31:03: Parsing message <xxxxxxxxxxxxxxxxxxxxxxxx\pd35002293159.msg>
 Wed 2009-06-03 15:31:03: *  From: xxx@xxx.com
 Wed 2009-06-03 15:31:03: *  To: xxx@yyy.com
 Wed 2009-06-03 15:31:03: *  Subject: RE: Another test email
 Wed 2009-06-03 15:31:03: *  Message-ID: <1EA8A9EC22947C4A9330C83196F53DB901BC6418@xxx.local>
 Wed 2009-06-03 15:31:03: Attempting SMTP connection to [yyy.com]
 Wed 2009-06-03 15:31:03: Resolving MX records for [xxx.com] (DNS Server: xxx.xxx.31.20)...
 Wed 2009-06-03 15:31:03: *  P=001 S=000 D=yyy.com TTL=(1) MX=[zimmail.yyy.com] {yyy.yyy.73.197}
 Wed 2009-06-03 15:31:03: Attempting SMTP connection to [yyy.yyy.73.197:25]
 Wed 2009-06-03 15:31:03: Waiting for socket connection...
 Wed 2009-06-03 15:31:04: *  Connection established (192.168.x.x:2496 -> yyy.yyy.73.197:25)
 Wed 2009-06-03 15:31:04: Waiting for protocol to start...
 Wed 2009-06-03 15:31:10: <-- 220 ******************************************
 Wed 2009-06-03 15:31:10: --> EHLO xxx.com
 Wed 2009-06-03 15:31:10: <-- 250-zimmail.yyy.com
 Wed 2009-06-03 15:31:10: <-- 250-PIPELINING
 Wed 2009-06-03 15:31:10: <-- 250-SIZE 20480000
 Wed 2009-06-03 15:31:10: <-- 250-VRFY
 Wed 2009-06-03 15:31:10: <-- 250-ETRN
 Wed 2009-06-03 15:31:10: <-- 250-XXXXXXXA
 Wed 2009-06-03 15:31:10: <-- 250-ENHANCEDSTATUSCODES
 Wed 2009-06-03 15:31:10: <-- 250-8BITMIME
 Wed 2009-06-03 15:31:10: <-- 250 DSN
 Wed 2009-06-03 15:31:10: --> MAIL From:<prvs=1405a1d17a=xxx@xxx.com> SIZE=4117
 Wed 2009-06-03 15:31:11: <-- 250 2.1.0 Ok
 Wed 2009-06-03 15:31:11: --> RCPT To:<yyy@yyy.com>
 Wed 2009-06-03 15:31:11: <-- 250 2.1.5 Ok
 Wed 2009-06-03 15:31:11: --> DATA
 Wed 2009-06-03 15:31:12: <-- 354 End data with <CR><LF>.<CR><LF>
 Wed 2009-06-03 15:31:12: Sending <xxxxxxxxxxxxxxxxxxxxxxxx\pd35002293159.msg> to [yyy.yyy.73.197]
 Wed 2009-06-03 15:34:02: *  Winsock Error 10054 Connection was reset by the other side!
 Wed 2009-06-03 15:34:02: Error writing to socket
 Wed 2009-06-03 15:34:02: This message is 61 minutes old; it has 0 minutes left in this queue
 Wed 2009-06-03 15:34:02: Remote queue lifetime exceeded; message placed in retry queue