[CentOS] What can I do to UNDERSTAND why I can't reach centos.org (but everyone else can)?

[Les Mikesell]

On Thu, Feb 28, 2013 at 4:52 PM, John R Pierce <pierce at 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.

-- 
   Les Mikesell
     lesmikesell at gmail.com