Again, please explain what connect to the internet with real IP means and what you do to connect to the internet. Do you bypass ISA server? Do you reconfigure your web browser? Do you disable the ISA firewall client?
Also, is your ISA server connected to a W2K AD network? IF so, are user accounts and computer accounts access to the internet controlled by Group Policy?
Another question is can you open the ISA server management console on the ISA server? If so, under Access Policy, IP packet filters, expand IP packet filters and you should have one for PoP3. Double click the PoP3 filter to open the properties sheet. Down at the lower left corner is an "Enable this Filter" box. That should have a check mark. Also verify that the filter type is predefined POP3 TCP IP protocol inbound fixed port 110 remote port=all ports. You also need to verify that on the local coupter page of the PoP3 filter property page that "Default IP address on the external interface" is checked OR that the "ISA server's external IP address is checked AND the correct IP address is listed in the box.
Another area to look at is how your user mail program [Outlook, Outlook Express, [or whatever mail program your using for PoP3 mail] is configured. Are the client programs configured properly.
Further, Has anyone made a change in Group Policy recently? IF so, did they change the Internet Users OU that ISA server uses to determine if a user or groups of users are allowed internet access from the LAN. You can view this group and/or change/add/delete groups/ OU's/or individual users from within ISA server. Site & Content Rules under Access Policy and Protocol Rules under Access Policy can be configured to allow/deny users/groups/OU's access to the internet. Verify that users/groups/OU's who are authorized internet access are listed here.
Further, You might want to just reboot restart ISA server services. Gods know but something just might have gotten stuck or an error happened and a reboot mi
