Running postfix on ubuntu, sending alot of mail ( ~ 1 million messages ) per day. loads are extremly high but not much in terms of cpu and memory load. Anyone in a similiar situation and know how to remove the bottleneck?

All mail on this server is outbound.

I would have to assume the bottleneck is disk.

Just an update, here is what iostat looks like:

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.00    0.00    0.12   99.88    0.00    0.00

Device:         rrqm/s   wrqm/s     r/s     w/s   rsec/s   wsec/s avgrq-sz avgqu-sz   await  svctm  %util
sda               0.00    12.38    0.00    2.48     0.00   118.81    48.00     0.00    0.00   0.00   0.00
sdb               1.49    22.28   72.28   42.57   629.70  1041.58    14.55   135.56  834.31   8.71 100.00

Are these numbers in line with the performance you would expect from a single disk?

sdb is dedicated to postfix.

I think it is queue shuffling, from incoming->active->deferred

More details from questions:

Server: Quad core Xeon(R) CPU E5405 @ 2.00GH with 4 GB ram

Load average: 464.88, 489.11, 483.91, 4 cores. but the memory utilization and cpu is minimal

Postfix instances between 16 - 32