HP OpenVMS Guide to System Security: OpenVMS Version 8.4 > Appendix A Assigning Privileges
Mailboxes are buffers in virtual memory that are treated as if they were record-oriented I/O devices. A mailbox is used for general interprocess communication. Unlike a permanent mailbox, which must be explicitly deleted, a temporary mailbox is deleted automatically when it is no longer referenced by any process.
Grant this privilege to all users of the system to facilitate interprocess communication. System performance is not likely to be degraded by permitting the creation of temporary mailboxes, because their number is controlled by limits on the use of system dynamic memory (BYTLM quota).