• 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.

This list is much less complete than the MAPS DUL^SM, but it does a good job, requires very little maintenance, and is freely available. The da.uu.net and ipt.aol.com entries alone cover a substantial portion of the Internet's dialup space.

To use with sendmail, simply include the current list of dialup zones in the access file with REJECT as the action.