A page has been recently deleted from the UPS web site, which included:
If I connect to UPS servers through a firewall, are there any known issues?
Because the response from the UPS server may show as unsolicited to your firewall software, you may need to configure your firewall software to accept messages from our unique range of UPS IP address ranges: “207.24.0.*”
At this writing, an in-site search on http://ups.com and a Google search still brings up the page reference, but the page is gone.