Declude Hijack allows your users to send out email easily while preventing spammers from relaying much mail through your mail server. This can be done without restricting users to specific IP addresses or requiring users to use SMTP authentication. The concept is simple: it works by only letting users send out a specific amount of mail in a given time period.
You will set 2 thresholds (which consist of a time period, and the amount of email allowed within that time period). For example, the first threshold may be 20 emails within 10 minutes, and the second threshold 100 emails within 30 minutes. Normally, all of a user's email will go out when they send it. However, if they reach the first threshold (20 emails within 10 minutes in this example), all subsequent emails are quarantined by Declude Hijack. If the second threshold is not reached (in this case, 30 minutes goes by without 100 emails being sent), then the mail will be sent. However, if they reach the second threshold, the mail will be moved to a permanent holding directory and will not be sent out.