Quoting David Wetzel (dave@turbocat.de):
> is it possible to block all hosts that have "dialup" in their full qualified hostname?

Yes, but...

As mentioned, RBLs work better.  I block (via /etc/mail/acccess)
dyn.attbi.net and some others like that.

> how?

You *could* write a rule to put in your .mc file (not .cf!) and
generate the .cf which looked for "dialup"
I might start with that in
LOCAL_RULE_0
R$*<@$*.dialup.$*>      $#error $@ 5.7.1 $: "550 dialup domains blocked"


But that's the free, 20 seconds of thinking, answer.  There may be conditions
you want to check for around it.