Hi Paul you have a PM
The error is
This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
XXXXXXX@blueyonder.co.uk
SMTP error from remote mail server after RCPT TO:<XXXXX@blueyonder.co.uk>:
host smtpin.blueyonder.virginmedia.com [XX.XXX.XXX.XXX]:
550 Administrative prohibition - unable to validate recipient
This now needs raising to IT as xocemp said way up there ^^^