Hosted Forefront Proection for External Exchange Server
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Title says it all really.  Is it possible to host a solution like Microsoft's 'Forefront Protection for Exchange Online'.  They say you just need to point your MX records to them.  So, how does an MSP go about putting a similar solution into effect when the client insists on keeping their Exchange Server where it is?  

Any tips would be awesome!

Thanks,
Justin