邮件类库为 MailKit
端口采用 465

邮件部分配置为

    client.CheckCertificateRevocation = false;
    client.ServerCertificateValidationCallback = (sender, certificate, chain, sslPolicyErrors) => true;
    client.Connect("smtpdm.aliyun.com", 465, SecureSocketOptions.SslOnConnect);

服务器上错误提示为

      MailKit.Security.SslHandshakeException: An error occurred while attempting to establish an SSL or TLS connection.
      This usually means that the SSL certificate presented by the server is not trusted by the system for one or more of
      the following reasons:
      1. The server is using a self-signed certificate which cannot be verified.
      2. The local system is missing a Root or Intermediate certificate needed to verify the server's certificate.
      3. A Certificate Authority CRL server for one or more of the certificates in the chain is temporarily unavailable.
      4. The certificate presented by the server is expired or invalid.
      5. The set of SSL/TLS protocols supported by the client and server do not match.
      6. You are trying to connect to a port which does not support SSL/TLS.

      See https://github.com/jstedfast/MailKit/blob/master/FAQ.md#SslHandshakeException for possible solutions.
       ---> System.Security.Authentication.AuthenticationException: Authentication failed, see inner exception.
       ---> Interop+OpenSsl+SslException: SSL Handshake failed with OpenSSL error - SSL_ERROR_SSL.
       ---> Interop+Crypto+OpenSslCryptographicException: error:14094410:SSL routines:ssl3_read_bytes:sslv3 <span><span>alert</span></span> handshake failure
         --- End of inner exception stack trace ---