Microsoft is warning users after a potentially unstable update was accidentally released for Exchange Server 2007.

The company said that a pre-release version of an update roll-up was inadvertently posted to the Microsoft Update and Windows Software Update services.

The update was intended to be released for Exchange Server 2007 Service Pack 1 after the company had further tuned it and fixed bugs.

However, the package was posted early and one of the bugs is causing major headaches for administrators who installed the update.

Microsoft explained that an issue with the Exchange Web Service component is leading the update to send some servers into a continuous crash cycle.

The company said in a blog posting that it expects to have a fix for the problem posted soon. In the meantime, Microsoft is recommending that admins uninstall the update and wait for the final release of the Rollup 4 package.

The software can be uninstalled through the 'Change or Remove Programs' screen within Windows.

The problematic update will be listed as version 1 of Update Rollup 4 for Exchange server within the program's support information window. To avoid confusion, the upcoming fixed update will be listed as 'version 2', said Microsoft.

"We apologise for any inconvenience and are working to make sure this does not happen again," wrote Exchange developer Scott Roberts.

"While at this time we are not aware of many customers being affected by this, we wanted to let you know in case you are currently testing what you downloaded two days ago."

The apology, however, is falling short for some users who had already installed the problematic update.

"This is absolutely unacceptable," commented one user. "If a pre-release patch can get into Microsoft update, you leave me no choice but to disable automatic updates."