If you're looking for a different Inbox Zero alternative, SaneBox is a good contender. It uses machine learning to categorize emails into folders you can prioritize, and it's got BlackHole for blocking spam, Snooze for deferring messages, Daily Digest for non-priority emails and Reminders for ones you haven't replied to. SaneBox has security built in with continuous testing and limited storage of just basic email metadata, so it's a good option if you want to keep your inbox organized.
Another contender is Ready to Send. This service composes human-sounding AI responses for Gmail so you can get your inbox under control. It offers fast response times, customizable responses and protection of your data privacy. It works on phone, tablet and PC, and supports more than 100 languages, so it's good for people with different needs.
If you like a tool that learns your writing style, Ellie is a good choice. Ellie composes responses based on the context of email threads and works as a browser extension for Chrome and Firefox. It supports multiple languages and can learn from your own emails, so it's good for people who want to automate email response chores.
Last, Superhuman has a rich feature set, including a split inbox, AI-assisted writing, follow-up reminders and read status tracking. It works with Gmail and Outlook and is designed to help you work more efficiently with an interface that's designed to be easy to use and with powerful collaboration abilities.