[exim] exim performance: how to send 10^6 letters daily?

Hello. At my work we are trying to use exim4 as a mail gateway for our
news mailing lists with currently about 1000000 subscribers, thus we
need to send 10^6 mails daily or better twice faster since we want all
mails to proceed before they became too old. Currently we are using exim
4.69-9 on Debian. After reading mailing archives we've enabled the
following options in exim configuration file:

split_spool_directory
queue_run_max = 100
remote_max_parallel = 200
queue_only_load = 8
deliver_queue_load_max = 12
smtp_connect_backlog = 50
smtp_accept_max = 500
smtp_accept_queue_per_connection = 1000

We are running exim process with -odqs -q1m options. We've put hints
database (/var/spool/exim4/db) on tmpfs. And in result on Intel
Corporation E7520 with 2 Intel(R) Xeon(TM) CPU 3.00GHz, with 4G RAM and
IDE software raid1 exim is not enough fast (we've managed to send 15 000
letters per 30 minutes) So the question is are there any hints how to
improve exim performance?

Currently it looks like our performance bottleneck is our discs -
currently we use software raid to improve performance, but still we have
only about 50-70Mb speed (as hdparm states). But if we put, say scsi
disks does this makes us much faster? Or should we use some different
mail server software (suggestions, please?)?

I found that to improve performance it was suggested to send to most
popular domains first but it's not very easy (not to say problematical)
in our setup.


Another problem is related to -odqs option. As I understand the manual
this option tels exim to do only routing and *no* delivery. Then why I
see the following messages in the log file:

2009-04-28 11:34:09 1LyhpP-0004TO-HX no immediate delivery: load average 9.40

? Probably this is our performance bottleneck too?


In any case thank you for any hints,
--
Peter.