Backup Exec 2012: Can Not Backup Exchange 2003 Reliably

I have like a 50% failure rate!

Backups fail because V-79-57344-766 - Cannot log on to MAPI with the specified credentials. Review the resource credentials for the job, and then run the job again.

V-79-57344-759 - Cannot extract mailbox messages from the Exchange backup. Review the job log for more information.

What up why can’t I backup exchange reliably?

I have been working with Tech support for 2 weeks and try this try that’s in not working