problema con postfix

[tony]

alguien le a ocurrido lo sgte, tengo 2 servidores de correo uno primario 
y otro secundario, ambos reciben bien el correo pero cuando quiero 
enviar desde el servidor secundario a mi server primario de correo me 
sucede esto o hacia el exterior

This is the mail system at host correo.ugdc.escopas.co.cu.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                    The mail system

<yusdelis@x.x.x>: host 192.x.x.x[192.x.x.x] said: 550
5.1.0
     <postmaster@x.x.x>: Sender address rejected: User
unknown in
     virtual mailbox table (in reply to RCPT TO command)