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Note to self: Extract TLS certificate from SMTP mail server


Replace the server name and the port. Usually the port will be 25.

 openssl s_client -connect <hostname>:<port> -starttls smtp

eg:

 openssl s_client -connect smtp.office365.com:587 -starttls smtp


Copy the output section of the certificate that looks like below.

-----BEGIN CERTIFICATE-----

###bunch of encoded text###

-----END CERTIFICATE-----

You can copy it and save it to a file with ".crt" or ".cert" extension and that's the mail server's certificate.
Make sure to include the -----BEGIN CERTIFICATE----- and -----END CERTIFICATE----- lines.

Note: OpenSSL is required.

Ubuntu: sudo apt install openssl

Other OSs: https://wiki.openssl.org/index.php/Binaries


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