resolv.conf file and DNS lookups

I have a qmail SMTP server that is one of our relay servers.  Everything for the past year has been working great.  We had a small issue this weekend with looking up mx records in order to send out an email.  

Looking at our qmail logs, a valid mx record was not found for a certain domain.  The reason why they weren't found is because our qmail server (using values from resolv.conf) was doing a lookup on our internal DNS servers first, where the mx records did not exist.  Once I added them, the emails shot out just fine.

My question is - if my resolv.conf file is setup the way it is below, and the mx records were not found internally, why didn't the machine move onto the other DNS servers (which are public) where the MX records DO exist?  It seemed like it stopped performing lookups once the records did not appear on our internal DNS servers.  

How exactly does Fedora move from one DNS server to another?  Am I missing something?

domain ourdomain.net
search ourdomain.net
nameserver      10.x.x.xx (internal DNS server 1)
nameserver      10.x.x.xx (internal DNS server 2)
nameserver      10.x.x.xx (internal DNS server 3)
nameserver      10.x.x.xxx (internal DNS server 4)
nameserver      6x.xxx.xx.xx (public DNS server 1)
nameserver      6x.xx.xx.xx (public DNS server 2)
nameserver      6x.xx.xx.xx (public DNS server 3)
nameserver      6x.xx.xx.xx (public DNS server 4)
JWeb AdminAsked:
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omarfaridCommented:
The dns query will stop once an answer is received (positive or negative). The fail over happen when the dns server is not responding. It is better to configure the internal dns server to query other public dns servers.

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JWeb AdminAuthor Commented:
that's what i was thinking - dns forwarding!  also, from what i've read, the resolv.conf file only supports 3 name servers
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