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Testing

Test SMTP with StartTLS via openssl

openssl s_client -starttls smtp -connect host.de:25

Test SMTP via openssl

openssl s_client smtp -connect host.de:465

Check certificate from domain

openssl s_client -showcerts -connect mydomain.de:443

Check which TLS version is used for SMTP

With the below command you can try to connect to a mailserver with a specific tls protocol. Possible values are: - tls1
- tls1_1
- tls1_2
- tls1_3

openssl s_client -connect myawesomeserver.de:587 -starttls smtp -tls1_2

Show certificate information as clear text

openssl s_client -showcerts -connect my.domain:443 | openssl x509 -noout -text

openssl x509 -in my.pem -text -noout

Show expiry or start date of certificate

Domain 

openssl s_client -connect mydomain.de:443 | openssl x509 -noout -enddate

File 

openssl pkcs12 -in my.p12 | openssl x509 -noout -enddate

SMTP 

openssl s_client -connect 1.1.1.1:25 -starttls smtp | openssl x509 -noout -enddate