[Exim] 550 Administrative prohibition

This is a multi-part message in MIME format.
--
[ Picked text/plain from multipart/alternative ]
Guys, I am having trouble here.

I am getting a bounce back message with a following error

  user@???
    SMTP error from remote mailer after RCPT TO:<user@???>:
    host domain.com [xxx.xxx.xxx.xxx]:
    550 Administrative prohibition

I have heard this problem for quite some time now and Google is not helping at all.

thanks

Molla
--