How to change message ID

Can anyone tell me where I can change the Message-ID going out in my mail headers?

Its currently:

Message-ID: 7B2F2846-AF19-4088-9142-5D108C7C3927@LocalDomain

and I need it to be:

Message-ID: 7B2F2846-AF19-4088-9142-5D108C7C3927@mydomain.co.uk

Thanks