In fact, Slack users have reported an average of 50% reduction of internal email (source: https://slack.com/results). But you're right, Slack is a closed system, and thus it cannot really replace email when used for external communications.
This is where Fleep comes in - a messenger that works with email, too. You can include anyone in a conversation with their email address, and if they're not a Fleep user yet, they can participate in the conversation via email. Fleep: https://fleep.io/
(Disclaimer: I do work at Fleep, and I love it :) )
Hadn't heard of Fleep before, but thought immediately that it might solve one use case for me.
After checking the pricing (good), if there was a self-hosted version (no, but maybe not a deal breaker) I looked for the Android app installer. Only available from Google Play Store. Unfortunately, that kills Fleep for me.
Please consider making your app directly downloadable and not reliant on Google Play Services.
I don't have Goggle Play Services installed on my phone and am not going to do so.
I understand why companies want their apps in the "Store" for discovery purposes. However, I don't understand why they won't provide an alternative direct download.
I do have F-Droid installed, but I appreciate companies may not want to open source their app. No problem, but I also won't be forced to install Google Play just to obtain it.
The Google Play store is also not accessible in China, an no doubt a few other countries, so it's just sensible to provide an alternative download anyway...
This is where Fleep comes in - a messenger that works with email, too. You can include anyone in a conversation with their email address, and if they're not a Fleep user yet, they can participate in the conversation via email. Fleep: https://fleep.io/
(Disclaimer: I do work at Fleep, and I love it :) )