1a.) For TLS, use the following command:
openssl s_client -connect smtp.gmail.com:587 -starttls smtp
1b.) For SSL, use the following command:
openssl s_client -connect smtp.gmail.com:465
2.) Both outputs of the preceding commands are long but you we are just interested in the entries that starts with:
-----BEGIN CERTIFICATE-----
and ends with:
-----END CERTIFICATE-----
3.) Copy that block of entries to a new file like smtp.gmail.com.pem.
This file is now the certificate file.
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