Microsoft has admitted considering replacing the maligned storage engine in its Exchange mail server software with its SQL Server database.

But "after much debate," the company had decided to stick with the existing Extensible Storage Engine (ESE) in Exchange, and issued a promise of a significant ESE performance boost in Exchange 2010, said Microsoft in a blog post.

"Yes! Some very smart engineers did amazing work and we had mailboxes up and running using SQL Server," wrote Microsoft. "It was ultimately determined that the best way to ensure we could drive compelling innovation into Exchange for 2010 and beyond was to remain committed to ESE."

Microsoft said it has been considering swapping out ESE, better-known as Jet, on and off for at least seven years.

Jet is often blamed by users critical of Exchange's scalability or performance. That has traditionally forced email administrators to restrict user inbox sizes or engage in complicated archiving schemes.

The rise of cloud-based email services, such as Google's Gmail, that offer unlimited inbox sizes at low cost and with minimal management requirements has put pressure on the market-leading Exchange to address its technical shortcomings.

At its Tech Ed conference this spring, Microsoft said Exchange 2010, using an upgraded jet database, will be able to read and write e-mails to disk 70 percent faster than Exchange 2007. It will also be able to store five times as many items per folder than Exchange 2007, and handle ten times more active connections.

Some analysts still predict that Microsoft will still switch to SQL Server in a future Exchange version, perhaps in the subsequent version slated to ship in 2013. Microsoft said it's keeping its options open beyond Exchange 2010, and that the company is only committed "to stay on ESE at this time."