SMTP in IIS – Can’t send to *.co.uk

I have recently set up an IIS SMTP server for all the mailing I do for a local band. It works perfectly for all domains except those ending in a .co.uk (or it appears so at least – I refuse this is just a co-incidence). I have set up *.co.uk as a remote domain under the domains branch (just as I have set up *.com *.net etc.) but its still does not work. Here is an example of one of the event logs:

Event Type: Warning
Event Source: smtpsvc
Event Category: None
Event ID: 4000
Date: 08/10/2002
Time: 15:09:46
User: N/A
Computer: BENTATHAM
Description:
Message delivery to the remote domain ‘whiskin.fsnet.co.uk’ failed for the following reason: Unable to bind to the destination server in DNS.

Data:
0000: d5 02 04 c0 ?..?

any ideas??