[Exim] Error Message

I have read throught the FAQ and cannot seem to find the answer to the
following error message.
Maybe someone can lend a hand.

:

  w.mcintosh@???:
    SMTP error from remote mailer after end of data:
    host mail.alloywheels.co.za [196.2.16.152]:
    550 Syntax error in 'To' header:
    "@" or "." expected after "Undisclosed-Recipient":
    failing address is:
    <Undisclosed-Recipient:;>

The strange thing is that I am receiving the same message for 8 different
users at 8 different domains.