Thanks both. We did use Mailchimp for a time. The issue with an externally hosted ESP is you have to have an unsubscribe link - even if you're sending within your own company. Tried to play around with that a little and rightly fell foul of their terms and conditions.
Is there a standalone bit of software that can connect to exchange that will do the job? Something like e-Campaign but better I suppose. Ideally a web based solution but without the unsubscribe requirement and direct exchange connection.