It is still a very competitive market out there and many messaging applications maintain a reasonably healthy market share. Microsoft Exchange however, is still believed to hold the largest installation base of all corporate groupware messaging products. Even in the growing hosted messaging services market, Exchange Server maintains a significant presence.
Ultimately, choosing a messaging server solution for an enterprise is about what tool is best for the job. Sometimes cost is the most important factor while other times the strengths of those who will be responsible for maintaining the tool may be the deciding factor. The answer may be different for different companies. Right now, there are no “Exchange killers”; however, there are some compelling Exchange competitors.