Re: [Exim] SMTP Spoofing - Preventing Local Relaying

Mail List wrote:

>
> Now with that account info if you try and send a message to a remote
> server, like for example mail to my yahoo account - me@??? -then
> the
> mail server will block the relay attempt and tell you "relaying denied"..
>
> --But-- if you try and send mail to another domain/user on the server
> housing domainA.com, then exim will accept the connection and hand-off
> the
> message..

Well, that's how smtp works-- basically. smtp is the way that mail goes
from one box to another. Say, for instance, you set your mail client:

smtp server: smtp.isp.com

then you send mail to smtp.isp.com, which looks up foo@???, and
sends it over smtp. What you're maybe missing is that your mail client
(outlook express) speaks smtp-- the same as anohter isp's smtp server.
Keeps thing simple.

So, following suresh's advice, block by IPs (or blocks of IPs-- in the
case of people who are dialing up)

any box who speaks smtp and can reach your box on tcp/25 will be able to
send mail to local users at your box. That's how smtp works.

You might pick up _Exim, the mail transfer agent_ by Phillip Hazel, the
author of exim, which has an excellent overview of the ins and outs of
how mtas work.

glen