Error: User 'User1' does not have an email address (when adding user to Meeting Workspace)

Hello Community

    When adding a user as an "attendee" to a "Meeting Workspace"
when you click in the checkbox under "Attendees", "New Item"
pops up on the Toolbar.

    After clicking New Item a dialog box appears prompting for Name
and comment.

    After typing a name into the textbox and then a comment, when
you click "Save" an error message appears stating:

        User 'John Smith' does not have an e-mail address".

    The users are in different forest/domain than the Sharepoint 2010 Server farm
and are connected via a one-way trust relationship.

    So the question is how do you get the Sharepoint farm to be aware of the
users email address in the other forest/domain.

    Shabeaut