I used a command like this one:
cd /root/letsencrypt
./letsencrypt-auto certonly -a webroot --webroot-path /srv/www/htdocs -d example.com -d www.example.com
Maybe I did not describe my problem exactly:
My server delivers sites over SSL (https://) - that is working without problems.
But on the client side in Thunderbird, in the menuitem preferences/advanced/certificates/properties I see, that the certificate from my server (running dovecot imap server) has expired.
Of course, I checked the path to the certificate in the Dovecot configuration.