PMDF User's Guide
OpenVMS Edition

In order to support IMAP's message unique identifiers (UIDs), for which the VMS MAIL message store itself has no satisfactory equivalent, the PMDF IMAP server uses auxiliary files, one such auxiliary file per IMAP hierarchical folder or actual VMS MAIL mail file. The auxiliary file for a VMS MAIL mail file will be named mailfilename-UIDDIR where mailfilename is the name of the underlying mail file. The auxiliary file for an IMAP hierarchical folder will always be named MAIL.MAI-UIDDIR and stored in the same directory as the hierarchical folder itself.