A blacklist is a collection of domains or email addresses that are denied access to a service. Blacklisting is a method of controlling abusive members who may otherwise disrupt business as usual for an Internet service provider.

For example, spammers are typically recognized by an IP or email address. If an e-mail host recognizes that an email sender is sending large quantities of unsolicited email, the host may choose to add the sender's IP or email address to a blacklist. To deliver an e-mail, a server will first check the blacklist for permission to complete the request. If the sender's IP or e-mail address is found on the list, the request will be denied.

Appalachian's domain will sometimes be placed on a blacklist.  When this occurs many companies will not accept mail from appstate.edu.