[exim] Does EXIM slow down when it gets fat?

In our normal day-to-day we have maybe 2,500 messages in the exim queue at
any time. We retry delivery every 15 minutes, slowing down the retry as
messages gets older.

Had a bit of a routing issue, where Exim was accepting mail but could not
deliver to the next server (not a mailbox).

The queue grew on one server to just over 200,000 messages ( 14 GIGS worth!
).

We stopped sending new mail to the server, so it could deal with what it
had.

I assumed it would blast through those messages very quick.

It didn't move quick, it seemed to crawl. Doing a tail -f command on the
log I could easily read the log it was moving so slow.

I ended up opening a bunch of SSH connections, picking a domain and doing
something like this:

for eximid in `exiqgrep -i -r @example.com`; do exim -m $eximid; done;

That got a higher number of messages processing, still with no load on the
server.

When the queue got down to maybe 90,000 messages I found the speed that
Exim was processing seemed to be increasing, started to time and see how
many messages it was doing per minute.

At 85,300 messages in the queue it did about 480 deliveries per minute.
At 67,775 messages in the queue it did about 785 deliveries per minute.
At 40,458 messages in the queue it did about 1,227 deliveries per minute.


I have read on here that Exim does not do well with a large queue, is my
observation normal? The bigger the queue the slower the processing?

How could I have handled that massive amount of mail automatically, I want
to see those messages flying!

Is there a way to configure Exim to stop accepting mail if there is so much
in the queue? Maybe if there is 75,000 message in the queue we just refuse
to accept anymore because there is obviously a problem and adding more to
the queue is just going to make things worse.