XYZ is a large real estate firm. Their core competence is their understanding of the real estate market, and their understanding of their customers and the customer needs. Their most critical tools are their CRM and email servers. They are growing, and so is their customer list and their IT and marketing costs are growing at an alarming rate. They currently use managed services to run their CRM, email, etc software but they have had to deal with significant delays as each growth spurt is bogged down by the time and cost it takes to provision new servers, install the software, test it and bring it online. Which cloud delivery model would be best suited for them?