where can I find and download the previous version before this terrible 5.8 version was released?
If you just download random apks from the internet, you can easily end up with one that is malicious. I have seen many modifications of my apks over the years that, for example, need more permissions than the original.
That’s why the thread linked above lists the GitHub releases (which hold the official apks) and F-Droid (which is a third-party that can likely be trusted).
5.806 is really a terrible one. I found the older 5.600 apk but there is no import of my data possible. I hope for 5.9 or 6.0 coming soon with a better item.
Try the K9 fork - OpenMail
Rather than getting a release from some random location, you can get past official releases at:
https://github.com/k9mail/k-9/releases
5.60 was the last pre-redesign release (currently on page 6 at the above url), if that’s what you’re after.
Here at removed or removed, you can find the apk file of your qurie.
EDIT by moderator: Removed links. Please don’t install APKs from a random website. They could easily contain malware. Official APKs of all prior K-9 Mail versions are available on GitHub. The link was already posted to this thread.