I’ve been a very satisfied user of K9 for years, but now I’m not able to use it anymore on my Samsung Galaxy Tab with Android 5.1.1 because of certificate errors.
My e-mail-provider told me that Let’s Encrypt renewed a root certificate and my system be too old to be able to refresh the certificate automatically. They asked me to download it manually from Chain of Trust - Let's Encrypt
Now the problem is that K9 requires a .pfx format to be imported but the Let’s encrypt page is only offering .der, .pem, .txt. I tried t convert a .pem using OpenSSL but failed with error messages. What can I do now?