There have been a number of posts on forums about the database limits in various versions of Exchange. This post is a quick summary.
The limits shown also apply to Small Business Server, which has Standard edition of the relevant version installed.
Exchange 2000 Standard Edition: 16gb *
Exchange 2000 Enterprise Edition: Unlimited **
Exchange 2003 Standard Edition RTM and SP1: 16gb *
Exchange 2003 Standard Edition SP2: 18gb soft limit, 75gb hard limit ***
Exchange 2003 Enterprise Edition: Unlimited **
Exchange 2007 Standard Edition: 50gb soft limit, unlimited hard limit ***
Exchange 2007 Enterprise Edition: Unlimited **
* Exchange 2000 and 2003 Standard edition SP1 or older, can both be increased to 17gb as a temporary measure to allow the store to be remounted to remove content.
** The actual maximum possible size of the Exchange database is 8TB, effectively unlimited.
*** Exchange 2003 SP2 and Exchange 2007 have soft limits. This is a setting in the registry that limits the database to the size shown. It can be increased as required.
The idea is to stop run away store growth. Should something happen within the Exchange setup that causes rapid store growth, it will hit this limit and be dismounted.
With this introduction of a soft limit, the way that the database limit is enforced was changed.
With Exchange 2000 and 2003 prior to SP2 the database limit was the physical size of the store. The amount of white space in the store was not taken in to account. This meant that once you hit the limit you needed to do an offline defrag of the database to get it below 16gb, even after you had removed content.
With Exchange 2003 SP2 and later, this behaviour has been changed. The limit is now the physical size of the store, minus the amount of white space. Therefore if you hit the physical limit of 75gb on Exchange 2003 SP2 then an offline defrag is not going to help. Users need to actually remove content to create white space in the store.
Furthermore the database size is not checked in real time. It is checked once a day, by default at 5am (this can be changed via a registry modification). Therefore if the store dismounts because the limit has been breached, you can simply restart the service and the store will mount again.
However in my experience, once you hit the limits, even removing the content will only be a temporary solution. Either an investment needs to be made in an archiving solution that actually removes the content from the store, or an upgrade to Exchange 2003 Enterprise edition or Exchange 2007 Standard needs to be carried out.
Relevant Links at microsoft.com
Exchange 2003 Limit Changes: http://technet.microsoft.com/en-us/library/aa998066.aspx
Exchange 2007 Limit Changes: http://technet.microsoft.com/en-us/library/bb232092.aspx
Increase the limit of the Exchange 2000 database to 17gb: http://support.microsoft.com/kb/813051