On Thu, Feb 28, 2013 at 4:52 PM, John R Pierce pierce@hogranch.com wrote:
They don't seem to parse headers, and go*past* the mailserver to the culprit.
you can't parse the headers until you read them, and you can't read the headers until you accept the incoming message. once you've accepted it, you can't bounce it back to the sending server via refusing the connection, and if you try and bounce it to the 'from' address you'll be spamming a lot of innocent parties who's email addresses have been forged on said headers.
so, if you use header parsing, all you can do is quietly drop the message.
That's not true - there are several phases to smtp delivery and you can reject at most of them. You just have to have a mailer where you can control the operations at the right place. Using sendmail with MimeDefang as a milter gives you about as much control as possible if you want to parse/scan headers and content and react accordingly.