Abstract: The invention provides a filter for filtering out spam mails before they arrive at a recipient's computer. Upon receiving a mail, the filter checks an associated insignia, e.g. the e-mail address of the sender, and if the sender is not yet in the allowed list, a reply mail is generated for the sender. When the sender responds to the reply, a priority is assigned to the insignia and the firstly forwarded message is held back until the insignia is selected to the allowed list. The filter could be implemented in standard computer systems. The invention further provides a method of filtering messages.