Hello,
I can´t send emails von my oc-server. I see this error: "Beim Senden der E-Mail ist ein Problem aufgetreten. Bitte überprüfe Deine Einstellungen. (Fehler: Connection could not be established with host smtp.strato.de [ #0])". In my log-file I found this entry "{"reqId":"SBs8P3LwyIUwO19De2TY","level":3,"time":"2017-12-13T15:08:06+01:00","remoteAddr":"xxx.xxx.xxx.xxx","user":"xxx","app":"PHP","method":"POST","url":"\/owncloud\/settings\/admin\/mailtest","message":"stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages:\nerror:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed at \/var\/www\/html\/owncloud\/lib\/composer\/swiftmailer\/swiftmailer\/lib\/classes\/Swift\/Transport\/StreamBuffer.php#94"}".
I have Let´s Entcrypt as SSL-Certificate-Service.
On my testing-server with the same owncloud-version a don´t have this problem.