pobox servers

Hello,

I am using pobox.com as an email forwarder.  Pobox mail is coming
from different servers.  When someone sends me an email, say [email protected],
greylist correctly adds it to the greylist. An hour later [email protected] 
resends the mail, but it comes through a different mail server, so 
greylist adds that to the greylist.  This could continue happening
and the mail may never get through.  Is there a way to rewrite 
mail coming from pobox.com to always be from the same server and ip?
Making greylist a bit less restrictive might also work.  Is there a
way to do this or possibly another solution?

thank you,

scott lair