RE: [cobalt-users] SMTP hole maybe - any ideas

["Dan Kriwitsky" <list1@xxxxxxxxxxxxxxxxxxxx>]

> I know I'd prefer to prevent this if possible, at least it 
> would allow me to ban the spammers smtp servers. The problem 
> is, to stop it would mean some form of pre smtp auth. I'm 
> sure I've read somewhere about using procmail to prevent 
> usage as described above. This does however also prevent for 
> example one user on a domain sending email using that server 
> to another.

Use a DNSBL that blocks dynamic IPs like SORBS or NAJBL. There's no way
to prevent mail from being properly delivered to domains that are meant
to receive email on your server from sources that are authorized. IIRC,
if you try to deliver mail to a domain on your server using a from
address of one of the domains on your server and that from address
doesn't exist, the server should reject the mail. That's how it's
supposed to work.

--- contacting host mail.example.com [123.45.67.89] on port 25
220 mail.example.com ESMTP Sendmail 8.10.2/8.10.2; Wed, 4 Jun 2003
11:46:05 -0500
> MAIL FROM: <george@xxxxxxxxxxx>
553 5.3.0 <george@xxxxxxxxxxx>... No such user here

> 
> I hope I'm on the right track, I didn't think the reception 
> of mail was the contention, rather the use of our own smtp 
> server to send the email.

Your SMTP server is not being used anymore than it would be no matter
how the mail was sent. I block a ton of *.dsl.example.com domains not
because I don't want dynamic addresses sending mail, but because those
dynamic addresses are constant sources of open proxies and spam.
-- 
C2003 Dan Kriwitsky

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