How to increase Exchange Server Database past the 18gb limit

After hitting the 16gb limit in Exchange 2003 a while ago, we decided to use the temporary 1gb extension registry key to let the mail store start again so we could clean up old emails & have a mail store again. 

When later we found out we could upgrade to SP2 and not have to worry as much (the db was forcing itself offline every few weeks), we researched and installed the patch.

Fast-forward to this morning, when I was surprised to find the mail store down again, this time due to the 18gb limit! A Quick google search led me to this article explaining that it's not an automatic increase, but rather the result of 2 registry keys, still being forced to a maximum of 75gb (a lot of mail).

You can configure a logical database size (logical size means the physical size of the .EDB and .STM  files minus the logical free space aka white space in each) limit for each Exchange database by creating a DWORD registry key named “Database Size Limit in GB”. This key should be created under the following location for the mailbox database and public folder database respectively:

• HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<Servername>\Private-GUID
• HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<Servername>\Public-GUID

If you wanted to set a limit of 40GB for the Public Folder Store, you would simply need to create the same key under the Public-GUID and configure it with a decimal value of 40.

After doing this, and restarting the information store service, you'll find an event in the Event Viewer's Application Log, id: 1216, with a message something like: "First Storage Group\Mailbox Store (MAILSERVERNAME)' is limited to [value entered, ie. 40] GB."