Postfix with MailScanner, Unable to send emails from a particular network

I have a mail server running on Centos 5.2, it is postfix 2.3.3 with MailScanner 4.77. When I send emails using this mail server as my SMTP relay, it works from my LAN but fails with my 3G modem.
here is a sample from maillog when it works (LAN)
Dec 8 17:54:00 mail postfix/cleanup[10411]: 9EFA72F0762: hold: header Received: from mynetbiosname (unknown [my.ip.addr.ess])??by mymail.server.com (Postfix) with ESMTP id 9EFA72F0762??for ; Tue, 8 Dec 2009 17:54:00 +0200 (CAT) from unknown[my.ip.addr.ess]; from= to= proto=ESMTP helo=

Dec 8 17:54:04 mail postfix/qmgr[7646]: 0DBFB2F0772: from=, size=38318, nrcpt=1 (queue active)
Dec 8 17:54:04 mail postfix/smtp[13376]: 0DBFB2F0772: to=, relay=mymail.server.com[ip.of.mail.server]:25, delay=4, delays=4/0/0/0.02, dsn=2.0.0, status=sent (250 M2009120817540328868 Message accepted for delivery)

With this, the email get sent.

On the contrary, this is only what i get when sending while am connected with my 3G modem.

Dec 8 17:40:09 mail postfix/cleanup[12370]: 4DB292F071F: hold: header Received: from mynetbiosname (unknown [ip.from.3g.modem])??by mymail.server.com (Postfix) with ESMTP id 4DB292F071F??for ; Tue, 8 Dec 2009 17:40:07 +0200 (CAT) from unknown[ip.from.3g.modem]; from= to= proto=ESMTP helo=

The log ends here, there is no more activities.

I use this server as an SMTP relay server, it doesn’t not host the sender’s mailbox nor the receiver’s.

I would really appreciate if someone helps me get this through.