Sendmail sends messages like root

Hi,
we are setup sendmail like a relaying. We are using user authetecing in the smart mail host. But when we use sendmail to send a message, the message is rejected, because user root, not is a user known in the domain.
Where or what must i do, to can be able to send messages from the sendmail server throught the other server. is it related with EXPOSED_USER parameter?

Thanks in advance.