If you have an internal DNS server, you should be using it 100% of the time. Just configure it to forward unresolvable queries to your ISP's DNS servers. If your DSL goes offline, you'll still have timeout delays for unresolvable hosts, but you can minimize those by tweaking your DNS server's timeout values to suit your client environment. (None of these suggestions address security, which should always be a concern in a DNS environment. At least with an internal server, you don't have to worry about someone spoofing your Exchange server by modifying your external DNS zones, assuming your network environment allowed connectivity to an external server.)
