Mail uses temporary storage in memory and on disk to improve access to data.
This allows the server to respond quickly when users retrieve or send their messages. You can adjust the settings if you have a lightly used mail server and want to free some memory, or heavy traffic and want to make the server more responsive. This setting specifies how much of the WebSTAR server's memory allocation will be used for the mail server internal lists such as which messages have been read, messages in a mailbox, whether the session is locked or not, and so on. This cache automatically purges old information as it needs the room. If you have a large number of concurrent users, which is most likely when they are using IMAP or Web Mail, increasing this cache size will improve performance.