MailScanner w/Postfix and Postgrey Question on rejected messages

[am.lists]

I use MS/PF/PG as a gateway.

I'm not sure if this is the right place to ask this question, but I
had a slew of messages not arriving today for a period of a few hours.
It's still too early to tell if they were rejected permanently or just
deferred and I should see them arriving later.

My config is as per the subject line...

My /etc/resolf.conf contains three DNS servers:

for discussion, they are

nameserver 1.1.5.5
nameserver 2.2.6.6
nameserver 2.2.7.7

(e.g. at least two of them are on separate networks...)

In the above example, server 1.1.5.5 went offline (crashed).

With only the first DNS server down, why did the appropriate piece
(again, not sure which application is responsible for managing DNS
lookups on domains) did not failover to the next nameserver on the
list?

If you guys redirect me to another list, I'll understand, but I'd be
very surprised if  I'm the first one to see this problem.

Best,
Angelo