Useful site with SSL commands, CSR and certificate decoder:

https://redkestrel.co.uk/articles/openssl-commands/

Display certificate request:


openssl req -noout -text -in /etc/apache2/ssl/server2016.csr

Display to whom it was issued from PEM file:


openssl x509 -noout -in imapd.pem -subject

Display certificate:


openssl x509 -noout -text -in /etc/apache2/ssl/server2016.crt

Create key:


openssl genrsa 2048 > www.linuxia.de.key

Create certificate request from existing key (SHA2):


openssl req -new -key www.linuxia.de.key -sha256 -out server2016.csr

Show public key information:

openssl rsa -in www.linuxia.de.key -pubout

Test SSL connection and print out SSL certificate:

openssl s_client -connect  192.168.26.241:443

Connect to SMTP server with STARTTLS:

openssl s_client -connect  192.168.26.241:25 --starttls smtp

For more information, check the OpenSSL Command-Line HOWTO.