One of the sites that I work with just implemented a feature on their site that allows members to invite their friends to the site by importing their address books from Gmail, Yahoo!, AOL, Hotmail, and other email providers. This is a common feature on most social networks these days, and there are many scripts available with this functionality.

As soon as they implemented the feature, they were sending tens of thousands of email messages per day.  Some of the big email providers did not take kindly to their email practices (which were completely legitimate) and they were blocked from both Yahoo! and Hotmail within a week.