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"