Google, meanwhile, has been making improvements to its platform "at a blistering pace," said Cain in his report, counting more than two dozen "substantial changes" to the platform in the past 12 months.

Those upgrades included improvements in and manageability, Gartner said. For example, new security features include the ability to reset a user's sign-on cookies. And enhancements to the system's manageability include the ability to manage policy by user groups.

Microsoft's answer to Google's upgrade pace "is that it intends to add functionality to Exchange in the cloud before adding it to the on-premises version," Cain wrote. "This represents a major shift for Microsoft, and its actual execution has yet to be demonstrated."

But Cain, in an interview, said a major reason why enterprises may reject Google is because their needs are too complex for a cloud-based offering. Their requirements in areas such as internal routing, application integration and compliance may be too much for cloud-based services to handle, he said.

"Someone with sophisticated and integrated systems isn't going to be looking to the cloud anytime soon," Cain said. "The cloud vendors can't be in the position of doing all this customization."