Repel for Outlook is a Microsoft Outlook add-in that provides effective spam filtering and begins working as soon as it is installed. Repel for Outlook activates whenever Outlook detects the arrival of a new email message. Repel examines the header information and compares the sender with an up-to-date public black list of known spammers. If there's a match, Repel for Outlook can mark the message as spam and move the message to the spam folder, move the message to the delete folder, or delete the message immediately. In addition to the public blacklist, Repel for Outlook features easy administration of both a private blacklist and a private white list.

The private blacklist causes messages to be filtered in the same manner as the public blacklist, but works for servers not previously reported as a spam source. The private white list is used to override the public blacklist. In addition, all of the lists can be manually edited.