Exchange Slow when secondary DNS server is offline

Hi,

I recently found that after a power outage my second domain controller (of 3) did not come back online. It was hung at a POST prompt. No big deal. I resolved that easy enough.

I noticed while the DC was offline that my Outlook connections to Exchange were painfully slow while that DC was offline.

That DC is the second DNS resolver for the Exchange servers and for clients at our main site.

I am curious as to why a DC that is essentially in place for redundancy would cause such an obvious problem with the Exchange Server client connections when it is not even resolving DNS queries for the Exchange server or clients?

Any thoughts on the topic are much appreciated.

Thanks

Chris