Muy buenos dias, amigos de Zimbra en nuestra organizacion tenemos montado un Zimbra Open Source version 5.0xxx, se nos a estado presentando el siguiente problema al enviar correos a algunos dominios,
a continuacion les muestro el error que sale al enviar el correo a uno de ellos.

This is the mail system at host mail.xxxxxxx.com.ve.

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

<creditoycobranza@xxxxxx.com.ve>: host xxxxxx.com.ve[66.xxx.xx.xx] said: 550
No Such User Here" (in reply to RCPT TO command)


Si me podrian dar una mano con esto, ya que e realizado varias pruebas pero no doy con el problema...