There is an update:
I checked /var/log/mail.log and found the following about the mail delivery. Is it because of the DNS settings ? Please help. Thanks.
Apr 23 01:14:31 mydomain postfix/smtpd[30871]: connect from nm14-vm0.bullet.mail.bf1.yahoo.com[98.139.21\
3.164]
Apr 23 01:14:31 mydomain postfix/trivial-rewrite[30875]: warning: do not list domain mydomain.org in BO\
TH mydestination and virtual_mailbox_domains
Apr 23 01:16:11 mydomain postfix/smtpd[30871]: warning: problem talking to server 127.0.0.1:10031: Conne\
ction timed out
Apr 23 01:17:52 mydomain postfix/smtpd[30871]: warning: problem talking to server 127.0.0.1:10031: Conne\
ction timed out
Apr 23 01:17:52 mydomain postfix/smtpd[30871]: NOQUEUE: reject: RCPT from nm14-vm0.bullet.mail.bf1.yahoo\
.com[98.139.213.164]: 451 4.3.5 Server configuration problem; from=<aoeuaoeu123@yahoo.com> to=<abc@mydomain.org> proto=SMTP helo=<nm14-vm0.bullet.mail.bf1.yahoo.com>
Apr 23 01:17:52 mydomain postfix/smtpd[30871]: disconnect from nm14-vm0.bullet.mail.bf1.yahoo.com[98.139\
.213.164]