Knowledge base
min-free-space<spool>set by the HubDirectiveWhen free space falls below the threshold, the directory stops accepting mail; when space comes back, receiving resumes by itself.deliver-only<spool>set by the HubDirectiveThe directory is drained by delivery, and new messages are not written to it; made for bringing up the spool of a failed server.priority<spool>DirectiveDirectories with a higher priority are always used until they fill up; with equal priority they alternate.delete-file-holders<spool>set by the HubDirectiveHolders are files the engine leaves in the spool directory to save I/O; the line decides whether to remove them after a message is delivered.