I had to laugh when I read this article on E-Week, entitled "Outages, bad updates hamper businesses."
Dana Gardner, founding analyst of InterArbor Solutions said most enterprises and SMBs (small and mid-sized businesses) deal with e-mail outages internally with some regularity, noting that the rates of uptime for these non-transactional apps can be as low as 97 percent and still be considered mission-critical. He told me:

So let's take a reality check on whether Web-based apps are or should be any better than behind-the-firewall apps such as e-mail that sometimes go down. That said, trust and value are essential for any migration to Web-based or cloud-provided apps. The bad news is that if Company XYZ has an e-mail outage, people go to the water cooler and chat about the Olympics and shrug it off. Few people outside the actual company know or care. But when Web mail such as Gmail goes down, it's for the world to see.
(emphasis mine - DWM)
Now I don't know about the rest of you, but if I saw my Zimbra server drop to 97% uptime I'd consider it seriously broken. I've had a Zimbra server up for the better part of a year, and total downtime (including the time I took it down to do upgrades) probably hasn't reached 24 hours yet. Take out the headaches I had on initial install/configuration and it's probably not more than 12 hours total.

Seems to me the industry is setting the bar too low. . .