Is it possible to deploy Exchange in any flavour as a relay without a DC?

You have to have DC on the site where you have Exchange.Gulab | MCITP: Exchange 2010-2007 | Skype: Gulab.Mallah | Blog: www.ExchangeRanger.Blogspot.com

Sorry for the stupid follow up, but is this a prerequisite of every exchange role?

Yes it is. You should have one DC and GC on the site. Kindly check the Prerequisite for Installing Exchange 2010 SP1: http://technet.microsoft.com/en-us/library/bb691354.aspx http://exchangeserverpro.com/installing-exchange-server-2010-pre-requisites-on-windows-server-2008 Gulab | MCITP: Exchange 2010-2007 | Skype: Gulab.Mallah | Blog: www.ExchangeRanger.Blogspot.com

As above mentioned, Exchange should be installed as a member machine in a domain. If there is no domain controller, the schema cannot be prepared. Thanks. Novak

The Edge Transport role (which can relay mail as you request) does not require access to Active Directory or a domain controller. Mike Crowley | MVP My Blog -- Planet Technologies

Does Exchange 2003 have this role if so can it route x400 messages?

NOOOOOOOOOOOOOOOOO! Cheers,Gulab | MCITP: Exchange 2010-2007 | Lync Server 2010 | Windows Server 2008 | Skype: Exchange.Ranger | Blog: www.ExchangeRanger.Blogspot.com

Does Exchange 2003 have this role if so can it route x400 messages? The Edge Transport Role does not exist in Exchange 2003, however one could be deployed with Exchange 2003 nevertheless: Deploy the Edge Transport Server Role in an Existing Exchange 2003 Organization Before Upgrading to Exchange 2010 Mike Crowley | MVP My Blog -- Planet Technologies