I have my exchange server 2003 and sometimes messages are queued in System Manager - Message Queue (local delvery not ready).
I usually restart the server and restart the SMTP services. It will be fine then but this has been happening alternate days. I checked the event viewer and noticed the following 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