How to remove WordPress word from confirmation email

when i add new user to my WPMU network I usually check the box “Add the user without sending them a confirmation email.” and once I click “Add New User” an email will be sent to that new user from WordPress not from my site? it is from wordpress@mysite.com and and titled new user registration so and so? how to set to be from my site name instead from wordpress?

please advice and thank you