Struggling with certificate

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?

You’ll need to install the certificate into Android itself, not in K-9. I don’t have an Android 5 device handy to verify the exact menus, but it should be something like this:

System Settings App > Security > Credential Storage > Install from device storage

Select the new certificate and confirm it. Then restart K-9 and see if it’s happy.

Thanks for your help, zootboy. Actually, I already had installed the certificate to android but forgot to mention it in my post. But then I tried to import it in K-9 because this K-9 dialog demanding a pfx file still was open. My error was that I didn’t restart K-9. Now I did and all works wonderful. :blush: