"Hotmail feels dated, it looks like a Honda from the 1990s," said Miller. But the service, while perceived as old-fashioned, continues to be a major Microsoft success: According to recent data from measurement company comScore, Hotmail led the market with 324 million unique visitors last month, compared to 290 million for Yahoo Mail and Gmail's 277 million.

Nevertheless, the reboot couldn't have come at a better time: comScore said Hotmail's numbers had dropped 4% in the past year, while Yahoo Mail and Gmail grew 2% and 17%, respectively.

"Most consumers are much more familiar with 'Outlook' as a Microsoft brand," argued Miller, who declined to speculate on Microsoft's motives for abandoning Hotmail. "They sometimes put time into branding, but other times they ignore it, so it's hard to gauge what's behind the name change."

Microsoft could have made the design and integration changes seen in Outlook.com to Hotmail, and kept the older nameplate and domain, Miller acknowledged. But it didn't.

"We're seeing a renewed interest from Microsoft in the consumer side, which has been stagnant for a long time," Miller said. "They have lots of cleaning up to do, but they're starting to realize that they have a diamond in the rough [in Hotmail] and that they should fix it."