[exim] Restricting the queue length [performance]

I need to send about 600.000 personalized messages under 1 hour (*not* a
spam, opt-in newsletter).

Exim queue grows large and IO seems to be the bottleneck.

Please advise configuration options to:
- deliver messages immediately (put in the queue only the "problematic"
ones)
- restrict the queue length (exim should ask smtp client to wait; I
develop the client and have full controll over it)
- any other ideas?

TIA!

Piotr Włodarek

PS I can easilly send 600.000 *non* personalized messages using BCC
field (1200 messages with 500 emails in each BCC) in just several
minutes, so network infrastructure is not an issue.