Microsoft Exchange Server reported error (0x8004011D): The server is not available

All workstations on a system I support can no longer send/receive email through Outlook 2003. This started late yesterday.

ERROR Message:
Task 'Microsoft Exchange Server' reported error (0x8004011D) : 'The server is not available. Contact your administrator if this condition persists.'

Event Viewer Error:
The MAPI call 'OpenMsgStore' failed with the following error:
The attempt to log on to the Microsoft Exchange Server computer has failed.
The MAPI provider failed.
Microsoft Exchange Server Information Store
ID no: 8004011d-0512-00000000

Operating System: Small Business Server 2003 SP1

All of the Microsoft Exchange Services are running and the Mailbox Store is mounted. The database sizes are: PRIV1.EDB - 10.5GB, PRIV1.STM - 6.2GB, PUB1.EDB - 215MB, PUB1.STM - 958MB. Total for all four files is about 18GB.

Is the 16GB limit per file or total? Will SP2 need to be loaded to increase the limit? Is there anyway to purge older emails since I cannot connect with Outlook 2003?

I am going in circles on this one - HELP!

I am tech savy, but I am an Exchange neophyte - so please be gentle.  :-)

Thanks