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ActiveInbox's paid version, which costs $25 per year, includes the option to tack notes to emails and attach deadlines to messages. The latter option helps keep track of daily and upcoming tasks in the to-do sidebar.
"It's actually a completely different way at looking at email," Mitchell says. "What we're doing is emails as tasks between you and your colleagues."
Until 2009, he had been running the inbox as a side project while working with another startup. After more than 700 people donated $20,000 to the project, he decided to start working on the project full time.
Through various beta versions of the product, the startup already has collected about 15,000 users, 1,500 of them paying customers.