Exchange with Terminal Services

Quick (maybe stupid) question. We are a small organization (35 people). Currently we run Citrix and share our applications on one server. Our e-mail is currently hosted by our ISP. We have started to discuss the possiblity of hosting our own e-mail via Microsoft Exchange 2007 for various reasons. However, my initial research has suggested we must be on Active Directory.

Is this correct? Thank you for any input.