Outages are becoming a distressing fact of life for Microsoft’s cloud e-mail customers, and users of other cloud services such as Google Apps. Two weeks of e-mail glitches plagued Exchange Online customers using Microsoft’s Business Productivity Online Suite (BPOS) in May. Office 365, the successor to BPOS which launched in late June, suffered an e-mail outage in August and then again last night and this morning.

Google Docs suffered an outage this week, and Amazon’s Elastic Compute Cloud infrastructure-as-a-service platform was plagued by outages and lost customer data in April and August.

The latest Microsoft outage was caused by what the company vaguely called a “DNS issue” and affected not just Office 365 but also the consumer services Hotmail and SkyDrive. The outages were spread throughout the world.

Taken together, the outages may put second thoughts in the minds of IT executives considering the move from locally hosted Exchange servers to Microsoft’s cloud, to Google Apps or to Amazon’s cloud.

Of course, IT systems can go down whether they are run by customers in their own data centers or outsourced to cloud vendors. But large institutions with multimillion dollar IT budgets may be able to achieve greater reliability by keeping IT in-house, without worrying about sensitive data residing in a vendor’s data center.

In response to the Hotmail and Office 365 outages, Microsoft tells Ars “On Thursday, September 8th at approximately 8:00pm PDT, Microsoft became aware of a DNS issue causing service degradation for multiple services. We achieved full service restoration at approximately 11:30pm PDT. We are conducting a review of our processes. We appreciate your patience.”

via Office 365, Google Docs go down again, could give pause to the cloud-wary.