Customers using Microsoft's Business Productivity Online Services (BPOS) suffered a multi-day email outage, caused by "malformed email traffic on the service," according to Dave Thompson, corporate vice president of Microsoft Online Services.

In a blog post, Thompson apologized for the outage and said that, as of Thursday night, the issues have been resolved and services are running smoothly. He also promised better communication in the future and a detailed post-mortem. (Hmm, remind you of a response to another recent cloud failure?)

BPOS is a bundle of hosted business services that includes Exchange, SharePoint and Lync. Exchange Online was scheduled for an upgrade on Tuesday May 12th, but users started noticing problems as early as Monday, reports Mary-Jo Foley with ZDNet. Microsoft had said the upgrade would not affect customers, but the troubles seemed to be specific to Exchange Online with users experiencing intermittent email delays of up to nine hours.

One of the most frequent complaints from customers was Microsoft's lack of communication. The BPOS health-status dashboard provides information and status updates about troubles in the system, but is available only to paying customers. Some customers called for the dashboard to be available to the public, and some reported such a delay in status updates that the dashboard offered little insight as to the actual problems.

"As a result of Tuesday’s incident, we feel we could have communicated earlier and been more specific," said Thompson in response. "Effective today, we updated our communications procedures to be more extensive and timely. We understand that it is critical for our customers to be as fully informed as possible during service impacting events."