You can try this (from the ISPConfig installation manual):
The ISPConfig comes with SpamAssassin, but without the Perl modules needed by
SpamAssassin. Run the following command after the installation of the ISPConfig:
If errors appear you have to install some Perl modules.
perl -MCPAN -e shell
install Net::DNS (when prompted to enable tests, choose "no")
To leave the Perl shell type
does not return any error everything is fine. You can then leave SpamAssassin by typing
" CTRL + c".