How to switch between SMTP servers at different locations on a laptop

There's nothing built in to Exchange 2003 or 2007 to do this for you. Exchange 2007 can be managed via Powershell, however, and the component that controls your smarthost is a Send Connector in Exchange 2007 and you could write a Powershell script that changes that attribute without too much effort. Whether or not you would want this to somehow run automatically or not would determine how complex this would need to be.

The first question is where does the failure appear? Inside the exchange organisation or in emails sent outside the organisation? So the first question is does the error appear for internal emails, external emails, or both? OWA is obviously an exchange component in its own right, and so the emails are accepted by the exchange system the moment you click send, so we can assume that you've got a basic working system of some kind set up.

For external emails, as you say this is where the problem lies, I'd be looking at the SMTP send connector(s) for the organisation. You've got a box installed with the exchange hub transport role, and you might have another box configured as an edge transport server.

So the question is do you have an edge server? And what queues are showing issues? Are you using the default address space scope/ email routing? Are you using a smarthost? Is there a firewall between any of these servers? This is where I would start digging for the problem.