The Mailbox messages that users have access to (whether linking a Mailbox message or viewing a message linked to the business process by someone else) will depend on their Mailbox permissions. By default, users can see linked Mailbox that reside in their personal Inbox and Sent boxes (either they have sent the Mailbox or were a recipient). A user who has permission can view the Project Mailbox folder that contains messages sent and received by other project members. For example, users with Project Mailbox view permission can see all Mailbox generated within a project.
Note: There are special conditions for linking Mailbox in the Request for Bid functionality. See Send and Manage Linked Mailbox from RFB Forms for more information.