Thanks IgorG,
I have followed the document to the letter, and things have changed.
Now I get the following when sending to this user.
Jun 19 10:26:52 XXX postfix/smtpd[1203]: NOQUEUE: reject: RCPT from XXX.co.uk[213.146.XXX ]: 550 5.1.1 <
[email protected]>: Recipient address rejected: User...