SUMMARY

A message is received into a mailbox where a redirection is configured to save a local copy, but the local copy is not present.

DETAIL

First validate whether the message was delivered to the store. Do this by checking the users mailbox file storage location (under their post offices MAILROOT) and check for the existence of the MAI file.
For further instructions on this see  http://www.mailenable.com/kb/content/article.asp?ID=ME020294

Assuming the file cannot be located, the following possibilities exist:        

1. Someone accessed their mailbox via POP and pulled the message from the server.
2. Their mailbox has exceeded its quota at that point in time. If this is the case, depending on the configuration, the message would have been delivered to the postmaster account or have bounced to the sender (but the redirect would have still worked). There would also have been an entry in the Post office Connectors debug log file as follows: Mailbox (%s) on post office (%s) has exceeded its quota.