1) I am attempting to implement a solution wherein I have a whitelist of senders whose mails go to the inbox (nothing new here). I don't know for sure yet, but I think it'd not be too hard to make maildrop work with my whitelist (?). (I know I can use whitelisting in SpamAssassin, for example, but what I am trying to do (see below) isn't necessarily related to spam scanning. So I figure the best placement for this is in the delivery agent. Am I wrong?)

2) The kicker is that I want all other (non-whitelisted senders) mail to be filtered elsewhere. I think if I can come up with a maildrop script that accomplishes #1 above, this may not be too hard, but I am concerned about two things:

a) Reading in a (possibly big) whitelist from a file or database during maildrop execution may not be efficient(?) and may be hard to code in maildrop script language

b) The FROM header is easily forged -- can I rely on typical postfix EHLO and client checks (such as requiring the client domain to match the sender domain or whatever) to catch those forgeries and safely be naive/trusting of what the FROM header says if I implement this in maildrop? Even if this kind of filtering was implemented elsewhere, it'd still have to be based on the FROM header, so maybe this question becomes irrelevant except to know how much postfix rules can help make the FROM header trustable....??