I recently discovered Permessa, a company that's been dealing with enterprise email for the last 15 years, with an impressive range of customers, everyone from Toshiba to the US Army Corps of Engineers. The CEO, Stefan Mehlhorn, runs an occasional blog covering email issues.
Their products analyze email and IM data as it flows across the internal network, focusing on automating common admin tasks like tracking down performance problems or enforcing email use policies. They've got some smart ideas on improving on the standard rules, like working out the impact of a message by multiplying its attachment's size by the number of recipients, rather than the usual blanket size limit. This makes it possible to send a 30MB attachment to a couple of people, but block a 2MB one that's going to 200 recipients, which is handy because the latter takes up a lot more room.
I can understand why their services are very popular at large companies. The performance and reliability of your email systems are insanely critical to running a business, so anything that helps optimize the messaging backbone is worth buying. Looking at the big picture though, it's depressing that we have to worry about 20MB attachments when a 1TB hard drive costs $200.
The web approach of throwing large numbers of cheap boxes at a problem has massive advantages compared to having a handful of super-pimped and expensive Exchange servers. You can actually take advantage of the cheapness of storage, rather than being stuck prioritizing performance over capacity as Microsoft recommends for Exchange. Individual boxes can fail and you don't have to go into Exchange recovery hell while your boss and all of your coworkers fume. As your requirements change, you can incrementally add or alter hardware.
Twitter's troubles make it clear that handling communication between large networks of people is still a really hard problem even with a modern cloud architecture, but just like the killer micros ate up the mainframe business, large networks of cheap servers are going to consume Exchange.