The message store is the component of the Messaging Server that contains the user mailboxes as well as the servers that provide IMAP, POP, and HTTP access to the mailboxes.
Unless otherwise noted, the message store (specifically, the stored process) should be up and running while performing management and maintenance tasks described in Message Store Command-line Utilities.
The size of the message store increases as the number of users, mailboxes, and log files increase. You can control the size of the message store by specifying limits on the size of mailboxes, by specifying limits on the total number of messages allowed, and by setting aging policies for messages in the store.
Depending on the number of users your server supports, the message store might require one physical disk or multiple physical disks. There are two ways to integrate this additional disk space into your system. The easiest way is to add additional message store partitions (see Managing Message Store Partitions and Adding Storage) Likewise, if you are supporting multiple hosted domains, you might want to dedicate a server instance to a single, large domain. With this configuration, you can designate a store administrator for a particular domain. You can also expand the message store by adding more partitions.