[ Fighting Spam
| Dialup Zones
| Blacklists Compared
| Current Blacklist Comparison
| Sendmail Configuration
Here is a list of DNS zones that
identify Internet hosts using dialup connections or other short-term
dynamically assigned IP addresses. Many mail system administrators
choose to refuse direct SMTP connections from these sources for
Thus, refusing direct SMTP connections from dialup computers forces
them to use their ISP's mail relay. This makes part of the spam
problem much easier for everyone (except the spammers, of course!) to
- Spammers use them because dialup connections are so inexpensive
and easy to get. Some professional spammers deliberately sign up for
several dialup connections with the same ISP, and when the one account
is shut down for violating the ISP's terms of service they simply
switch to the next account, from which they may be able to continue to
spam for hours or even days before the ISP catches up to them again.
Lather, rinse, repeat.
- Blacklisting individual dialup addresses is a waste of effort,
because the spammer can simply hang up and immediately dial in again,
which will usually give them a different IP address.
- Virtually all ISPs that provide dialup service also provide a mail
relay for their dialup customers to use. This lets those customers
quickly hand their e-mail to a local mail server that will deal with
the complexities of e-mail delivery such as resending in the face of
temporary failures. The log records kept by such a mail relay also
make it easy for an ISP to positively identify their spamming
customers who use it.
To use with Sendmail, simply
include the current list of dialup
zones in the
access file with
This document was last updated by
Jeff Makey <firstname.lastname@example.org> on
14 August 2012.