[Mimedefang] Checking for a valid sender
Steffen Kaiser
skmimedefang at smail.inf.fh-bonn-rhein-sieg.de
Tue Nov 18 10:02:55 EST 2003
On Tue, 18 Nov 2003, Alan Madill wrote:
> A large percentage of spam has a fake from address. Has anyone
> tried using md_check_against_smtp_server to test the senders
> address?
Bark.
Think about what are you doing here:
>
> sub filter_sender {
> my ($sender, $ip, $hostname, $helo) = @_;
> $supposedsender = $sender;
> $sender =~ s/[<>]//g;
> $sender =~ /^(.*)\@(.*)/;
> $supposedserver = $2;
> return md_check_against_smtp_server(
> "<postmaster\@mydomain.com>",
> $supposedsender,
> "mail.mydomain.com",
> $supposedserver);
> }
>
> I haven't tested it yet and the perl wizards can probably get that
> down to 2 lines. :-)
Each time you receive one email, you prentend to sent a mail back to the
server of sender.
Assume, the other server has the very same code applied, hence, it tries
to verify _your_ mail delivery. Hence, it opens a connection to your
mailserver.
Now you get another SMTP connection ....
This scenario might become better, if you use "<>" as Sender.
However, as far as I know you must know the MTA handling this domain,
meaning: MimeDefang does not resolve MX records; that's why many of your
verifications will fail. The function was introduced to verify _your_ own
recipients with _your_ own mail server (e.g. Exchange), for instance on a
mail relay / firewall system that does not know about local users.
Bye,
--
Steffen Kaiser
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