Oct 11 07:26:56 mail postfix/smtpd[20306]: NOQUEUE: reject: RCPT from convo.abc.com.my[]: 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
The server with address is in the same LAN with server
The postconf mynetworks show mynetworks =
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.