My experience is with BES 4.0.x, but I've seen the 20301 error in the following situations:
1) BlackBerry user has been "hidden" in Global Address List (GAL).
2) Exchange server is inaccessible to BES, or the user's mail store has dismounted.
3) BlackBerry user's mailbox is moved between mail stores on same Exchange server, making it inaccessible to the BES until the Agent is restarted.
The first one can be detected/corrected in ADUC.
Usually the second case is easy enough to see, as you're flooded from calls from users that share the same Exchange server or message store.
To test the third case, use the IEMSTest.exe utility in the /Utility folder for the BlackBerry installation. If the mailbox is accessible using the test utility but the 20301 errors persist, restarting the BlackBerry services seems to resolve the issue.
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