In epn.conf, specify the purpose of the cert/key is for client (user) authentication, i.e. not for secure SMTP delivery.

       smtp_client_cert <certificate>
              Specifies the path to a single file in PEM format containing
              the certificate. Default None.

       smtp_client_key <private key>
              Specifies  the path to a file containing the private key in.
              Otherwise the private key will be  taken  from  certfile  as
              well. Default None.

       smtp_client_key_pass <private key password>
              Specifies  the  password for decrypting the private key. De‐
              fault None.

1. Specifies the path to a single file in PEM format containing the certificate.
2. https://docs.python.org/3/library/ssl.html#ssl.SSLContext.load_cert_chain
3. Default None (empty value).
4. smtp_client_cert =

1. Specifies the path to a file containing the private key in. Otherwise the
2. private key will be taken from certfile as well.
3. https://docs.python.org/3/library/ssl.html#ssl.SSLContext.load_cert_chain
4. Default None (empty value).
5. smtp_client_key =

1. Specifies the password for decrypting the private key. It will be ignored if
2. the private key is not encrypted and no password is needed.
3. https://docs.python.org/3/library/ssl.html#ssl.SSLContext.load_cert_chain
4. Default None (empty value).
5. smtp_client_key_pass =