how to prevent fake email to enter my domain?

[Jorge Costinha]

i receive a lot of spam mask as internal email, as if someone within my network send it, but in fact is just spam, from outside.
i figure i could solve effectively just with spam.whitelist.rules and spam.blacklist.rules, for some reason i can't understand it only works with some emails.

here what i did, whitelist is checked first, if rule matches it skips the remaining rules and blacklist aswell:

on spam.whitelist.rules:

From:	192.168.10.	yes

on spam.blacklist.rules:

From: *@mydomain.com  yes


any internal email will match the rule on Whitelist, so it never gets rejected in blacklist. if its fake, it wont match the ip address and it only matches the blacklist.
when i did this, almost all fake email was detected, however some aren't. is there something i miss in here? is there a better approach? 
also regarding SPF records, mailscanner doesnt handle SPF? 

thanks!  
__
Jorge Costinha
-- 
This message has been scanned for viruses and
dangerous content by HCC MailScanner, and is
believed to be clean.

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