IPSwitch IMail 8.1/8.2 SMTP server has an Access Control list option in the SMTP Security tab. They don't have a tool that makes it easy to do bulk importing of IP network blocks. To make matters worse, the file where the Access Control option stores addresses is in a binary format, as smtpd32.acc, making it nearly impossible to just edit manually.
This script uses documentation from IPSwitch to generate a new smtpd32.acc file for your installation of IMail 8.1 in the proper format from the popular email blacklist sites, Okean and Blackholes.us. You could also create your own custom list of network blocks to enter into a custom smtpd32.acc file if you put it in either the Okean or Blackholes.us file format. Simply use this script to generate a new smtpd32.acc file, place it in the main IMail installation directory (C:\IMail for a default IMail installation), and restart your SMTP service.