pub async fn build_client(
smtp_config: &SmtpConfig,
client_builder: SmtpClientBuilder<String>,
) -> Result<(SmtpClientBuilder<String>, SmtpClientStream)>pub async fn build_client(
smtp_config: &SmtpConfig,
client_builder: SmtpClientBuilder<String>,
) -> Result<(SmtpClientBuilder<String>, SmtpClientStream)>