It appears to be hardcoded. Having said that if you were to add DiskXtender@ to an existing user in AD the email may appear in the user's inbox as having come from that user.
Hope this helps.

Thanks Frank,
Yes, I thought so. It is hardcoded. Already check at registry
HKLM\SOFTWARE\Legato\DiskXtender\Alerts, I only find MailServer entry and keys for every alert list. I am just waiting confirmation from EMC employee or affiliate for this hardcoded "stuff".

Regarding your suggestion, unfortunately, our domain/mail admin does not agree with adding DiskXtender@ to existing account.

BR

You may have to start a support call.