I have kind of my own email server. It uses letsenctypt certificates. Usually I check and send my mail using Thunderbird and have no problems, but I tried to use Kmail because it is better integrated into DE. I get email without problems but I am unable to send an email. I get error: "The server failed the authenticity check (koshka.ddns.net). The issuer certificate of a locally looked up certificate could not be found No certificates could be verified"