This filter implements greylisting, allowing OpenSMTPD to temporarily
reject sessions of clients it has not seen before. Unlike many
implementations, this one is SPF-aware so it will properly handle
greylisting for domains doing relaying through multiple MX, as long as
they publish a valid SPF record.

The filter currently supports:

* IPv4 and IPv6 address greylisting
* SPF greylisting
* startup whitelisting of IP and domains
* automatically renewed whitelisting
