Google has the policy of allowing engineers to devote 20% part of their work hours to personal projects.
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In 20% me time, Gmail was created by Paul Buchheit as a side project.
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Google launched Gmail on 1st April 2004, many people thought this is an April fool prank.
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In 2004, Hotmail by Microsoft was the email service provider with 30 million active users. So Gmail didn't perform well till 2012.
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But Google didn't lose its hopes and they keep innovating new things, Today Gmail is dominating the consumer email market with more than 1.5 billion active users.