Lars Wirzenius's procmail filters on steriods, with attitude, known as spamfilter, basically enforce this sort of policy, though in a slightly more friendly manner.
A white, black, and grey list are set up. Textfiles with email addresses or domains listed. You are responsible for adding to your black and white list. The grey list is automatically generated from addresses you send mail to.
Blacklisted mail is, at your option, dumped, denied, or filed as spam. Whitelisted mail goes on to your personal mailbox or additional filters. Everything else gets dumped to a "nopasswd" box. You can decide what you want to do with it there. I've created a set of shell scripts which extract mail headers, check for previously white- or black-listed addresses, and/or add the address to the appropriate list.
The distinction is that "grey" mail isn't rejected, it's just accepted at lower priority. The recipient is responsible for categorizing senders, not the sender.
Word of caution. spamfilter has an extremely fascistic attitude toward spam, and tends to assume hostile until proven otherwise. It also has a rather aggressive anti-spam autoresponse message. Mailing lists must be filtered correctly or they are treated as spam. I've more or less been forced into resigning one mailing list subscription due to poorly configured filters (and a touchy list admin...well, OK, he's also the primary author of the software discussed on the list, and it was his post I'd rejected <g>).
I have to say that the system works pretty well, I get about 300-500 emails daily, and see virtually no spam. The filters are a bit mysterious in their workings and I'm still trying to tune them appropriately. procmail is not the friendliest interface.
Recommended for geeks, newbies might prefer a more forgiving alternative.
Karsten M. Self
SCO -- backgrounder on Caldera/SCO vs IBM
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