[Exim] Sending question

Hi all,
When the users on our virtual server send mail, the recipients receive
it from "Nobody [nobody@???] on behalf of user@???"
. This happens when the mails are viewed in Outlook.

In other mail clients such as Eudora, the Sender portion in the mail
headers shows as Nobody [nobody@???].
What do I do to correct it.
Thanks,
Sangeetha Naik