Wietse Venema: > sb: > > Our point of view is plain: are we dealing with "proper" e-mail servers? > > Good question. > > > Our emphasis, therefore, is on the DNS, to identify the sender and > > its MX RR, because it is the de-facto standard to say "this is > > where I receive e-mail". > > For the envelope sender address, the sending IP addresses are covered > by SPF policy. Spammers were among the first to adopt it. > > > How else could you tell them apart? > > Reputation (DNSXL).
Additionally, some people have used the check_policy_service feature to collect additional information about the remote SMTP client. See http://www.postfix.org/SMTPD_POLICY_README.html for the protocol. Wietse
