Why relay access denied error although within same LAN

[POla]

Oct 11 07:26:56 mail postfix/smtpd[20306]: NOQUEUE: reject: RCPT from convo.abc.com.my[201.33.192.11]: 554 5.7.1 <freshgrad@yahoo.com>: Relay access denied; from=<deliver@xyz.com.my> to=<freshgrad@yahoo.com> proto=ESMTP helo=<returnofconvo>

As shown in above, the convo.abc.com.my relay to use deliver@xyz.com.my to send email to freshfgrad@yahoo.com, is it right?
Does this error happen because of the fact that mail.xyz.com.my do not relay for convo.abc.com.my?
The mail server, mail.xyz.com.my is with external IP 202.33.192.11.
The server with address 201.33.192.11 is in the same LAN with server 202.33.192.11.
The postconf mynetworks show mynetworks = 127.0.0.0/8 192.168.210.128/27
Two server are within the same LAN by default zimbra allow the relaying. Why now cannot?
The weird thing is that only yahoo mail cannot sent, others can send out successfully.