The maintenance and management of a company's email servers has become a major expense and an endeavor with major consequences and costs of failure. Something that most employees take for granted, or consider to be as simple as a telephone, is actually a major cost and potential headache. After all, email is like a telephone, but imagine moving the entire telephone company into your company's server room and paying them out of your own pocket. Not very efficient, really.
Wouldn't it be better if email management was more like the telephone company? You pay your share, and the necessary maintenance and engineering is accomplished off-site by experts and professionals. Your business reaps the benefits, and the headaches are handled somewhere else, by people whose job it is to solve the problems that cause those headaches.
This is essentially what email outsourcing does for a company. It removes the most troublesome elements of handling your own servers and frees up staff to devote time and resources to more important projects.