How do I send from the other domain?

Greetings,

My client upgraded from Mail free to Mail Lite because the first feature listed is "Email hosting for multiple domains". I have now added a second domain. I tried to add a user on the new domain, but I got an error message saying "License limit reached". I don't know the difference between a user and a license.

Now that there are two domains added, how does he send emails from the new domain that was added?

Thanks,

Shane.