Well then as suspected they are resolving the wrong IP address. You need to contact their ISP and ask them why they are ignoring the A records at the nameservers for this domain.

Usually this happens when someone's ISP was also that person's webhost.

To the Layman, the ISP is still hosting the DNS, and instead of telling that person's computer to go on the Internet to look up the address, it checks with the ISP's nameservers first. If the ISP has a zone it gives that person the local results instead of what the Internet has to say.

Ask the ISP to remove the zones for this domain if the scenario above applies to you.

If that is not the case, then you should try to clear the local DNS cache. This is best done by closing all programs, especially the email readers, and better yet, just power down all the machines on that network, and reboot the router, then power the machines back up.

You can also do much of the DNS cache clearing from the command line, but this is a quick way to get it done without getting into technical specifics.

If rebooting the router doesn't solve the problem you'll need to contact the ISP or network administrator to figure out why they are checking for the domain locally instead of on the Internet.