Private Messages module is an addon to program which has quite big usage scope: as classifieds software, online inventory management system, business/community portal, yellow pages script or even light content management software (CMS). Complete internal messaging system solution allowing members to communicate keeping email addresses private. Members can be notified by email if they get new private message. You can define maximum mailbox size for each member. If you want paid membership website you can set it to allow replying to ads for paid members only. Also it's possible to allow free replies to paid member messages, so that paid members will always get responses from non-paid users.