Very slow Mailbox backup

I am using Backup Exec 10.0 to backup mailboxes on Exchange 2003 on Windows 2003 Server, Standrad Edition.

The throughput is about 1.0 MB per minute. It takes about 7 hours to backup a single mailbox containing 400 MB of emails.

I have enabled Single Instance backup for attachments.

For database & file backups, the throughput of the same program is about 150 MB / minute.

Any ideas why exchange is so slow to backup? It has been like this for a few weeks…

Thank you.