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The capacity limit for a Microsoft Exchange Server server-based Information Store is 16GB. This could be all in one folder, or split up among many folders. A Microsoft Exchange Server generally hosts one Private Information Store (16GB for all Mailboxes), and one Public Information Store (16GB for all public folders).
Personal Folder stores have a theoretical maximum of 2GB total storage. This could be all in one folder or split among many folders. The limit to the number of messages (objects: messages, folders, files, etc.) in a Personal Store is 16K.
A Microsoft Exchange Server can be configured as a private only, public only, or public/private Server. There is a limit of 16GB on any Microsoft Exchange Server for the private database (DB) and a different 16GB limit for the public DB. Therefore, a public/private Microsoft Exchange Server can house 32GB of storage, 16GB of which is for the private DB and the other 16GB for the public DB.
If you have a large Public Folder, you can dedicate a single Microsoft Exchange Public Folder Server to house the data for that one Public Folder, so the limit for the size of a single Public Folder is 16GB.
Microsoft Exchange performs single instance storage in the database on a Microsoft Exchange Server, so the 16GB limit on the private database is really a much larger logical storage. Single instance storage means that if 1000 users on a Microsoft Exchange Server get the same message, the message is only stored once in the database and each of the users gets a pointer to the message. The same single instance storage also holds for attachments. If a user opens their copy of a message and modifies it but they don't modify the attachment, Microsoft Exchange doesn't resave the attachments, just the new message body.
The Microsoft Exchange beta sites saw at least 3:1, and sometimes upwards of 6:1, single instance storage rates because most messages have an average of between 3 and 6 recipients. To extend this, the logical storage limit for users on a single Microsoft Exchange Server is at probably least 16GB*3 and possibly upwards of 16GB*6.
Another factor to consider is how many people use Personal Folders (.PST files) as their delivery store. If users are using Personal Folders, the Microsoft Exchange Server can handle more users. The time at which the limit is reached for storage in the private DB on a specific Microsoft Exchange Server will depend on the pattern of usage by the users.
By default Microsoft Exchange sets up circular logging on the Directory service and the private and public DB's. What this means is that as the transaction log files get flushed to the databases, they are deleted. The beta sites saw, on average, no more than 4 logs files, each of which was MB. So in the default install configuration, log file growth will not be a factor. If an administrator configures their Microsoft Exchange Server not to use circular logging, the log file growth will be based on the number of transactions that are occurring on the Microsoft Exchange Server. This is based purely on activity.