A beta provider - PlumMail
 

Plum Mail is an alternative to email that uses your existing email address.

Do so much more with your correspondence than traditional email or instant messengers allow.

Plum Mail is an open system. You can write to anyone.

https://plummail.co/

Personally, I rather like email the way it is for the most part. On the other hand, chat systems are horrible! So, I don't like the idea of combining chat functionality with email. From their website it is hard to figure out exactly what Plummail does that is so revolutionary.

Their lack of modesty is so revolutionary:

Watched the video. It is more suitable for when you need to work together in a team, not for the individual e-mail user.

And, I suspect most teams that want or need a chat type system at work are already using Slack or Teams and it will be hard to get them to switch. I worked at a small tech company for awhile, and despite the fact that we were an email services provider management was always trying to get us to use various chat type systems for internal communications. What ended up happening was that we ended up with multiple parallel communication channels instead of using the new one because people from the outside were always sending us regular emails that would then get incorporated into an email stream. So, I can see the idea as possibly being a good one, except in the real world we were able to use existing tools in email to make it work well for us. The universality of email makes it hard to beat for business or personal communication.

Is this what Fastmail wants to achieve with Topicbox?