New domain but email cannot be delivered

[#1]

Hello
I have registered a new domain and now have two domains and my usual account.
Emails go to and from my usual account and my first domain OK.
I can send email using the new new domain name - but emails to the new domain name are returned saying
5.1.0 - Unknown address error 550-'#5.1.0 Address rejected ****@********.co.uk'
yet the set up mirrors the first domain name.
What could be wrong?
Geoff

[#2]

Locked in favour of duplicate post http://community.plus.net/forum/index.php/topic,86769.0.html