[exim] setup Exim to improve SMTP performance

Hello,

sorry for this "frequent" question, but I don't really found what I'm
searching.
So, we actually have 6 small servers (sempron 2.2Ghz, 512Mo RAM, 1 IDE HD)
to send a newsletter once a week to about 400'000 members.

We setup Exim 4.5 (Debian Sarge) on each of them, but can't send more than
280 emails per minute (less than 5 per second). It seems slow, no ?

The spool dir is in a XFS or JFS partition (dependding of the server),
and the
hints database is in memory (tmpfs).

Have I to set anything else to improve perf ? Does another hardware really
improve this ?


Thanks in advance,
Olivier