You cannot edit the SUBJECT line or the contents of the emails being received as notification from BEWS
in order to have a common sender email i would recommend the following
Create an AD account with an active and unique mailbox
The requirement for a unique name comes from the MAPI limitation for Exchange backups
if you are not backing up exchange resources you can ignore the UNIQUE part
Make the account a member of the Domain Admin group
Configure all backup exec servers to use the account as the default,system and service account