Most people like to use Gmail. But some people prefer something that especially suits their preferences. K-9 Mail is something that can achieve this. Although it is still not up on the Play Store, it promises a mail application that is tailored to one’s preferences. It is an open-source client and has interoperability which makes it the best alternative for Gmail. It can also handle multiple accounts.
The last time an update happened was in 2018. Now they have released a set of new features in their recent update to 5.8. many changes including the removal of keyboard shortcuts were introduced. The device on which the app runs should have an Android 5 or higher. The user interface has also been changed drastically. Autocrypt setup message and encrypted subjects support have also been added. It has also fixed many bugs.
You may be able to find the update soon on Play Store. The Play Store now has the 5.6 version.