Both the admin proposal page and the customer's proposal page have a Messages tab: a chat-style thread grouped by day, with a Reply box and Send button (messages post instantly without reloading) and a Mark As Read button that marks the other side's messages up to the latest one as read; read messages show a double check mark. Unread messages from the other party are shown as a red badge on the tab. Every message sent is recorded in the Activity log as Message sent. Customers can only post on proposals they are attached to; the thread is not available on New/Archived proposals since customers cannot open those.