Delaying Delivery For Large Subscriber Lists

I have a qmail + ezmlm set up for a list server that hosts about two
dozen moderated announcement lists.  None of the lists have really
high amounts of subscribers; mostly between 500 - 2,000 members. I
would guess each list gets maybe one or two messages sent to it a
week.  Not a huge load on the server, but enough of one that keeps it
busy.

Anyway, we just got a client that wants a list of 10,000 subscribers
from addresses he's collected over the past 10 years he's been in
business.  I'm assuming most of the addresses are really stale and
will purge themselves out.  My concern is that if he sends to his
list, the queue will get loaded up and if any other client tries to
send to their list, their moderate message will have to wait until
ezmlm rips through the addresses in the first request.  This delay
could cause me some unnecessary  support calls if they don't get their
moderate messages quickly.

My first thought would be to find out a way to accept the moderate
messages yet delay the actual delivery of the messages until later in
the evening allowing future moderate messages to get sent out in a
timely fashion.  Could anyone offer some suggestions on what I might
be able to do?  Any links to what to look at?  Any intelligent search
terms to start researching?

Thank you.

-ken schweigert