HubSpot auto creates a contact for anyone that emails in - or is cc'd into a reply in conversations. For some customers who manage through support, this will mean a large number of contacts can enter the database. The only way to prevent this at the moment is by setting filtering rules, as all filtered emails do not create contacts. There is a use case from a customer in this post..
This is a request to build a "Create contact" rule giving us the option to disable automtically creating contacts for people we communicate with in Conversations.