1. Mail domain name (the host part of the sender's e-mail address) must have an MX records;
   2. Mail domain name and the connection host address must be resolved;
   3. Connection host must be listed at the one of nameservers which hold the sender's mail domain name.

The second from the developed algorithms is a greylisting like algorithm "Check Delays" which is based on statistical check of delivery delays, generated by a [tempfail] return code, that allows to exclude a direct receiving of a mail, which not passed through the standard mail server.
NOTE: Using this feature the delivery of the mail from unknown, but legitimate senders will be delayed for a count of 5-minutes checks, which setted by the "maxcount" configuration parameter.
And the third of original algorithms is "auto-confirmation" algorithm: The sender of a "mail contact" (sender-recipient pair) means as authenticated (whitelisted) if the filters statistics module has registered the both of a sended forward (request) and a received backward (reply) messages of this contact. The messages from the such authenticated senders will never be delayed by the previous greylisting algorithm.

Первый алгоритм реализуется средствами постфикса, второй обычным грейлистом(gld, spamd и т.п), третий настраивается уже в спам-фильтре.