Our company supports a number of Exchange servers for our clients and up until recently one of the bug bears we had was the hard encoded 16Gb database limit.
This meant that as soon as the combined total of the users email hit 16Gb, Exchange would shut itself down. There is a way to temporary increase this limit to 17Gb to allow you to delete emails and run the defrag utility to free up dead space but as soon as the server has been rebooted it's not long until it hits 16Gb and the problem returned.
Since these days 16Gb is not a massive amount of data, users send and work with large attachments in some of the industries we deal with and it started to become an issue. However, Microsoft in a rare moment of actually listening to its customers decided to increase this limit to something with a little more headroom.
Hence, from Exchange 2003 Service Pack 2 you can set the limit to up to 75Gb. Microsoft decided that having a simple tab in Exchange System Manager to set this limit would be too easy, you have to set it via a registry hack.
Once you install Service Pack 2, the limit is set at 18Gb and you have to choose a new limit depending on storage space you have.
The Keys that need to be changed are for the Private mailboxes:-