A remote mailbox is a type of mailbox in Exchange that is associated with a user account in a different Exchange Server organization or environment. It enables a user in one Exchange organization to have a mailbox in another Exchange organization. Remote mailboxes are commonly used in scenarios involving mergers, acquisitions, or cross-forest migrations. They provide a way to maintain mailbox access and data for users across different Exchange environments while preserving separate organizational boundaries. The remote mailbox facilitates communication and collaboration between users in different Exchange organizations, enabling them to send, receive, and store email messages and other mailbox-related data across organizational boundaries.