After installing Exchange SP1, no mails were delivered and the following errors appeared in the event log.
Initialization of external interface POP3 failed; Error
Initialization of the IMAP4 external interface failed with error
Initialization of external interface IMAP4 failed; Error
Microsoft Exchange OLEDB was unable to initialize because of differing
versions of store and exoledb dll.
I fixed it stopping Exchange Services and reaplliying SP1.
More info on: http://support.microsoft.com/?kbid=919076