SSL certificate information

openssl x509 -text -noout -in

Check certificate at Dovecot
openssl s_client -connect <>:993 -showcerts

check with url
echo | openssl s_client -servername <> -connect <>:443 2>/dev/null | openssl x509 -noout -dates
my_url=URL_I_NEED_TO_CHECK ; echo | openssl s_client -servername $my_url -connect $my_url:443 2>/dev/null | openssl x509 -noout -dates
note:
URL without https part

create csr request
openssl req -new -key /PATH/TO/SSL_KEY -sha256 -out HOSTNAME.csr

check Certificate Signing Request
openssl req -noout -text -in HOSTNAME.csr

Cert key matcher
openssl pkey -in privateKey.key -pubout -outform pem | sha256sum
openssl x509 -in certificate.crt -pubkey -noout -outform pem | sha256sum
openssl req -in CSR.csr -pubkey -noout -outform pem | sha256sum