If you want a user to appear to send from a secondary domain you must configure the user's profile within webmail.
Logon as the user, then select options from the list of icons across the top of the page.
Select the Personal Information link, then the Edit your identities link. Click on the drop down list Your identities: and select default. Enter the required name and reply email address then click the create button. This will then create a profile for the user with the just entered reply email address.
Howard