HP OpenVMS Guide to System Security: OpenVMS Version 8.4 > Appendix A Assigning Privileges
The PRMMBX privilege lets the user's process create or delete a permanent mailbox by executing the Create Mailbox and Assign Channel ($CREMBX) system service or the Delete Mailbox ($DELMBX) system service. The privilege also allows the creation of temporary mailboxes with the $CREMBX service.
Do not grant PRMMBX to all users of the system. Permanent mailboxes are not automatically deleted when the creating processes are deleted and, thus, continue to use a portion of system dynamic memory. System performance degrades as system dynamic memory becomes scarce.