Life is not so easy for spammers that they can just send all their
email twice. Graylisting, and the like, require you to send the email
from basically the same IP address/block and use a consistent domain
name and such. After a few hours, other anti-spam tools such as
DNSBLs, RHSBLs, DCC/Pyzor/Razor, etc. have had a chance to react.
Do you imagine what is typical spam-bot? It is simple automate.
It gets list of targets and template of message. Make them send the
same message to same target with same envelop twice as simple as
add "wait 30min; replay once;" to execution script.
If you looking to make better condition for your "DNSBLs, RHSBLs,
DCC/Pyzor/Razor", you have just to put every mail coming to your
domain in some kind quarantine system for the same 2 hours and check
this cage regulary.
Everybody get his satisfaction: you get 2 hour delay for your user mails
with better spam detection chance and every other server on internet do
not suffer additional load created by your server anymore...