Outlook 2007 & 2010

We are running Small Business Server 2003 Premium edition, ISA is installed on the server. We use Exchange. If we received HTML e-mails when we are at the office on the network, Outlook 2007 & 2010 do not download the images, but if we are off the network and download e-mail via a 3G data card the html images download in the mails. How can a person get Outlook to download the HTML images when connected to the network?