Take a look at MailSite: htpp://www.rockliffe.com/mailsite.htm
We have many customers that have migrated from Post.Office. We have a couple of
unlimited versions depending on your needs and it integrates straight out of the
box with Radius servers such as RadiusNT and Emerald. It's been in the
marketplace for two years and we have lots of satisfied ISP customers.
Regards, John.