[cobalt-users] e-mail alias not working

["Richard.Dix" <mail@xxxxxxxxxxxxxxxxxxxxxxx>]

e-mail alias not working
Hi 
I have set up a new site www.domain.co.uk, and though the usernames will
accept e-mail, alias added in the panel will not and I get the following
error. 
) 

----- The following addresses had permanent fatal errors -----
<r.stan@xxxxxxxxxxxx>

----- Transcript of session follows -----
554 5.0.0 MX list for domain.co.uk. points back to
ns.positivecomputing.co.uk
554 5.3.5 <r.stan@xxxxxxxxxxxxx>... Local configuration error.

I have tried removing the site and users and re adding them to the system ,
but still no go. 
Any ideas? 

First post here. Thanks in advance.

Richard D