"We had to make it such that we're managing a sizable amount of information offline and doing it well in a way that's seamless to the end-user," he said.

For example, in Gmail, users can put labels on messages, as well as tag them with stars to indicate their importance, and Google can use that information to determine which messages to download. Sheth estimates that in most cases Gmail will download several thousand messages, preferring those that are more recent as well. Depending on the amount of messages users have on their accounts, they may get downloads going back two months or two years, he said.

Google will begin to roll out the Gmail offline functionality Tuesday evening and expects to make it available to everybody in a few days, whether they use Gmail in its standalone version or as part of the Apps collaboration and communication suite for organizations.

While the feature was "rigorously" tested internally at Google, it is a first, early release upon which Google expects to iterate and improve on. That's why it's being released under the Google Labs label. Users are encouraged to offer Google feedback.

Users have been able to manage their Gmail accounts offline via other methods for years, since Gmail supports the POP and IMAP protocols that let people download and send out messages using desktop e-mail software like Microsoft Outlook and others.