[exim] Exim outgoing emails throttling

I normally sends mass no. of emails via script twice per day (around 40000
emails) each email contains one recipient.
And my script will sleep for 5 seconds after every 1000 emails. I noticed
many exim processes running when script exeuctes
and around 20-30% emails stucks in queue and server load is quite high
during mass mailing.
What are the specific parameters to optimize outgoing mass emails ? Is
there any way to throttle outgoing emails ?

Furhter, i noticed stuck(queued) emails logs describes.

* Recipient address rejected: Greylisted for 2 minutes
* 451 Temporary local problem - please try later
* R=dkim_lookuphost defer (-1): host lookup did not complete

To me it's look likes recipient addresses are not accepting my emails.
Probably DNS, RDNS or SPF issue. right or wrong ?

and for successfull outgoing emails normally log describes
* SMTP Outbound Connection ..
what does that means ?