Most email providers maintain list of names and addresses of companies that have been negatively connected to spam in the past. Emails of those senders are being blocked so users do not have to be bothered unnecessarily. Furthermore, there are blacklists which include users that do not want to obtain any advertisement. Blacklists can exist locally or on central servers as so-called real-time blackhole list (RBL). Depending on the purpose, there are also hybrid types.