sendmail virtual domain

[Jessica Zhu]

I'm sure someone asked this before. Only I haven't found the clear answer 
about this.

I need to set up a sendmail virtual host for one of our client. The 
virtual domain is theirname.org. I can recieve the emails to this domain 
now. My question is if I want certain users' out email looks like from 
theirname.org, what should I set in sendmail.cG file? 

Thanks for the help!

Jessica