To resolve the issue, please refer to the following technote:
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
If it does not help, please send the exact error from the job log, the version of Backup Exec and the version of Exchange you are using.