ISPs blocking SMTP connections from dynamic IP address space

[Bill Campbell]

On Mon, Jun 07, 2004, Lucas Holt wrote:
>
>
>Just make sure they are truly dynamic ips.  Many people block ips identified
>as "DSL" connections.  Those are not necessarily dynamic ip based....

Some of the largest ISPs in the country, including AOL, are blocking what
they consider ``residential dsl'' in an attempt to stem the flood of spam
and worms that are propagated through owned Microsoft Windows machines on
broadband connections.  The majority of spam today is sent through
zombified Windows boxes that either are open proxies or have spammer
software installed on them that ``calls home'' to the spammer's servers to
get spam and lists of addresses to deliver.

Several months ago I installed a Linux server at one of our customer sites
running postfix on a QWest dynamic DSL line, and found that AOL was
blocking their SMTP connection with an immediate message saying that they
refused connections from ``residential'' DSL connections, and disconnecting
immediately without presenting an SMTP header.

I redirected all the traffic to AOL through one of our mail servers here
using the postfix ``transport'' mechanism, and had the customer order the
smallest fixed IP block that QWest offered.  As soon as that block was
working, their server could connect to AOL's servers without a problem,
leading me to believe that AOL and QWest are co-operating to distinguish
between the dynamic and fixed IP blocks.

Bill
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