If Exchange is running sp2, it's possible to change the data-base size from 18go to 75go

To be able to store 75go in the database, you have to add a registry key in :

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\\Private-013e2e46-2cd7-4a8e-bfec-0e4652b94b00

type : REG_DWORD

Name : Database Size Limit in GB

Value : 75

Concerned product: Exchange

Information source: http://technet.microsoft.com/en-us/library/aa998066.aspx; http://technet.microsoft.com/en-gb/library/bb232092.aspx