To temporarily increase the size of the database over the 16GB limit you need to do the following:
1. Stop the SMTP service (this prevents new mail from being delivered while you fix your database size problem. Failure to stop SMTP could result in your database expanding beyond 17GB.)
2. Start regedit (you might need regedt32 instead?)
3. Go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<Exchange Server Name>\Private-<hexadecimal string>
4. Choose Edit > New > DWORD Value.
5. Name the new value “Temporary DB Size Limit Extension” and press Enter. (Casesensitive)
6. Double-click the new key name.
7. Enter a value of “1″ in the Value data box
8. Click Decimal in the Base box
9. Click OK.
10. Close regedit.
11. Mount the information store.
12. Delete data you don’t need, such as old mailboxes, or export a large mailbox to a PST file. Just get the store as far below 16GB as you can.
Now look for the following in the event log an event 9657 which says you are using the temporary limit. You know it when you see it.