There's been plenty of blogging, twittering, and general hand-wringing about Google's Gmail outage Tuesday. But rather than extend this into yet another philosophical discussion about the viability of cloud computing, let's keep this in mind: Businesses who've signed on for Gmail don't expect perfection. In fact, both Google and Microsoft only agree to 99.9% uptime for their online email offerings.That means that for up to 43 minutes each month, Google and Microsoft (which-surprise-also has a huge and growing customer base for hosted email) could have unavailable email service and be in compliance with their service level agreements with paying business customers.

Now naturally, they'd lose customers in droves if that were to happen. But other than a few errant months in the past few years, Gmail has usually been well above 99.9% availability each month. The point here is that businesses using Gmail and Exchange Online do not view email as mission critical. Therefore, email doesn't have to be up 99.99%, 99.999%, and certainly not 100% of the time. And, in turn, these businesses get a pretty cheap email service.

What do you get for almost-but-not-quite-perfect email service? At Google, you pay $50 per user per year for the full Google Apps suite, no negotiations, no volume discounts. That works out to $4.17 per user per month. At Microsoft, the cost is $10 per user per month for just Exchange online, and $17 for the full online business suite, including Exchange, SharePoint, texting and meetings. In either case, you can add or drop however many users you want each month.

So Microsoft is a little more expensive than Google. But, Microsoft is willing to give up a bit more if its service falls below 99.9%. Google gives you free days at the end of your contract (which could be a few years from now), Microsoft actually reduces your monthly bill. Example: Yesterday, paying Gmail users got three free days at the end of their contracts. If the outage had happened at Microsoft, users would have gotten a 25% discount off of their September bill. Below 99% availability, it's half-off, and below an unsightly 95% availability, your service-as it should be-is free for the month. (And lucky Microsoft; if Exchange Online goes down, the news may not make it past its paying customer base, while the whole world knows when Gmail crashes).

CIOs know they're agreeing to an imperfect world when they sign contracts with Google and Microsoft for online email. And if Microsoft and Google wanted to make their email 99.999% available, they would build more hardware and redundancy into the infrastructure. But, in their cost analyses, they've weighed how much to put into the infrastructure versus how much customers are willing to tolerate for cheap monthly email service. Believe me; if the world starts clamoring for "five nines" on monthly email subscriptions, they're going to cost a lot more.

No, this isn't so much about whether cloud computing is ready. It's about tradeoffs, weighing the cost and management benefits of onsite versus offsite, and whether the world comes to an end if we can't check our email for several hours each year.