How do I change the name of sender "root@domain.com" ?

[RobVGZ]

I need to change the sender's name root@domain.com to another user's name like user@domain.com. I'm using MMDF, but I can't find settings to do this.
I want to start using SCO Openserver 5.05 to send mailings to customers, but they should be able to reply mails back to another user. In fact root@domain.com doesn't exist in the outside world and we want to keep it that way.

Thanks,
Rob