A spam detection system can monitor incoming and outgoing email messages and prevent email messages from being delivered. This spam detection system incorporates a sender ranking system that maintains prior sender's email addresses and an associated reliability value in a database. If an email message is categorized as spam, the system searches to see if the sender is located in the database. If the sender is located in the database and their reliability value is above a certain threshold, the sender's reliability value is decreased and the email message is treated as not spam. If the sender is not located in the database, the email message is discarded as spam. If an email message is not categorized as spam, prior users located in the database will have their reliability values increased, while new users will be entered into the database at a default level.