Secondary DNS question...

[SH Development]

Our secondary DNS machine went down (and unnoticed for 24 hours).

Today, we had multiple people calling about email that hadn't come in, and trouble with outgoing emails not going out.

Our primary DNS was up the whole time.  So my question is, why would my secondary being down, and only my primary being up cause so many problems?  I thought the whole idea behind having two DNS servers on different networks was to never have a failure like this.

My understanding was that when DNS is queried, the one that responds fastest is the information that is used.  If the secondary is down, then the primary would by default always be fastest (and only).

I think I reasonably understand basic DNS and the setup, but this has me thinking that something isn't set up right.

Can anyone shed any light on what might have happened here?  Could my primary not be responding as it should?  All the tests I have run on it show that it is responding normally.

Jeff