ActiveInbox is rooted in David Allen's Getting Things Done methodology--it started out as "GTDMail"--and helps you turn email messages into tasks so you can get to inbox zero. It requires you install an add-on to your browser that lets you process and sort each message with a few mouse clicks.
When you read a message, you have the option of designating it an "action" if it requires you to do something or "waiting on" if you need someone else to take action. In the former case, you assign it a due date, add the required action, and archive it. It's then moved from your inbox to a folder with all your other action emails, which you can access whenever you want to tackle those tasks. Similarly, emails that require others to act will be moved to a "waiting on" folder, where they'll be tracked until the person replies or you decide to track them down.
ActiveInbox's Personal plan gives an individual full-feature access for one Gmail account for $3.30 per month. The Professional plan bumps that up to three accounts for one person for $5 per month. For teams, the Business plan adds multiple accounts, a team admin panel, and one-on-one support for the team leader for $9 per month.
While ActiveInbox is a sound solution to email overload, it's not a quick fix. It requires you to adopt a new set of habits, and developing them takes time. But if your job is email-driven, ActiveInbox or at least the workflow it's built on--is probably your best bet for long-term control of your inbox.
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