Exchange 2010 cannot push mail to mobile devices

Hi Guys

I've installed exchange 2010 and everything looks fine except that when i configure mails on mobile devices, it configures fine but mails dont get pushed to device. OWA works fine and mail flow internal works fine. When i connect my mobile phone on LAN it does recieve mail, but does not work when im on external network.

Please assist what the problem might be.

Thanks in advance