Microsoft is also warning of another issue: Users who have the Microsoft Office Outlook Connector installed to synchronize Hotmail with Outlook will see an error message after they install the Google plug-in.

The error message, which warns that "some functionality will be disabled," is triggered by another registry key that the Google tool modifies and that is used by the Outlook Connector. "When confronted with this error message, we recommend that users click 'Yes' to enable functionality. The Outlook Connector will then continue to function normally. The Outlook Connector will reset this registry key to the correct value," reads Microsoft's posting.

Last week, several industry analysts recommended that enterprises do their homework regarding the new sync tool, because it can't fully replicate in Gmail the experience of using Outlook with the Exchange server.

The Burton Group's Guy Creese pointed out in a blog post that the Google plug-in currently doesn't support Outlook features like tasks and notes, and that IT managers should tread carefully if they have special filters and custom templates in their Outlook deployments. "For workers wedded to using every feature in Outlook, the Google solution is still insufficient," Creese wrote.

In a research note, Gartner analysts Matt Cain and Tom Austin said that companies that use Exchange and are considering moving to Apps' Gmail should first do a "stress test" of the sync tool and then do "a widespread pilot."

"We expect that Sync for Microsoft Outlook will have incomplete functions and that users will resist moving from Outlook to the Gmail client, thereby pressuring Google to fill in the functional gaps," they wrote.

While these functional gaps shouldn't automatically disqualify Gmail, IT managers should carefully consider them while making a decision, Cain and Austin said.

In an interview last week, Eron Kelly, senior director of the Microsoft Business Online Services Group, said Microsoft expected Exchange users to be dissatisfied with the feature gap when using Outlook with Gmail.

"This will just further reinforce in the eyes of customers and users that the Outlook experience is best fully replicated feature by feature in Exchange," Kelly said.

Kelly invited Google to consider becoming a hosted Exchange provider in order to get around the feature-gap issue. "We'd be happy to help Google do that. We have a great partner program."

Regarding the view that Google is increasing its competitive pressure on Outlook and Exchange, Kelly noted that Google is but the latest vendor in recent years to support Outlook connectivity, joining Novell, IBM, Oracle and Sun, among others.