We are looking for a provider to host a secondary MX in the UK, It will be designed to act as a failover for 500 days for a maximum of 10 days whilst a replacement primary MX is sourced.

This service would need to stay live all of the time and we need the following:
UK Based
Spam Assasin
Managed Service with SLA and 24 hour support cover.
The Ability ro relay a specific domain to a specific IP address.

We would be willing to add new domains and IP addresses if we are shown how to do this. All of the management and maintenance of this service would need to be completed by the company supporting it.

Can anyone make any suggestions as to who can do this?
We have very little experience with Linux and this is why we would like to outsource this.