Hello,
The error message that .....is a corrupt file can occur due to various reasons, large subject lines can also cause this. Refer to the folowing technote for m ore information on the same:
Title: When backing up Microsoft Exchange 5.5 / 2000 individual mailboxes, the message " WARNING: "?Top of Information Store?? " is a corrupt file. This file cannot verify." is returned in the backup log as an exception in Backup Exec
http://support.veritas.com/docs/255047
Hope this answers your question. If the problem persists, revert with details.
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Regards.