I tried it...it's absolutely and completely free :)

All the features, all the members, crazy stuff.

Run team retrospectives easily, quickly, and absolutely FREE GoRetro is an agile sprint retrospective tool making the entire retro process seamless, simple, fun, colorful, productive, and unlimited.

Knowmail supports localization per demand

Client based plugin connected to Knowmail's SaaS in order to generating the heavy modeling for each user

Machine learning is a type of artificial intelligence providing computers the ability to learn without being explicitly programmed.

It is also automated filtering and prioritization, as well as its aimed for corporate with individuals receiving hundreds of daily emails, thus cannot filter them semi-manually.

Furthermore, there are next-best-action features, as to save time, keep organized and reduce noise...to move to appropriate folder automatically, clean an email until a later (more appropriate time), mute yourself from a conversation until named, etc.

Email data is not shared, as it remains client side, where recommendations are given per user.

No identifiable data leaves your device, and only anonymized info is utilized to build a "personal model" for a user.

It uses machine learning in order to build a personalized model for each user, based on their personal habits and email communication behaviors

Its UI is like Outlook as it works on top of Outlook, keeping employees communicating by email same as they regularly are.