Milter-greylist

Hello,
I'm using milter-greylist for quite some years. I have a probably simple 
question. I found that many Spammers use  SPF records so whitelisting 
mails from MTAs with SPF records does not seem a solution to me. 
Therefore I have nospf for years in my config. On the other hand there 
are many big mail hosters sending from a whole bunch of MTAs. When they 
get greylisted, the next attempt will come from another MTA so there's 
another tupel IP, from, to and it gets greylisted again. In sum this 
sometimes leads to good mail getting delayed for hours or days. How to 
come around this problem?

I did not really find a documentation what the different values for 
"spf" would mean. man page is rather short on this.

I want mail from one sender to one recipient get greylisted only the 
configured timespan - regardless from which (spf verified) MTA the 
connection attempt comes. I'm quite sure this is possible - just do not 
know how...

Thx,
Carsten