You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: tokio1_smtp_tls.rs
    path of file: ./repos/lettre/examples
    
    the code of the file until where you have to start completion: // This line is only to make it compile from lettre's examples folder,
// since it uses Rus
    
async fn main() {
    tracing_subscriber::fmt::init();

    let email = Message::builder()
        .from("NoBody <nobody@domain.tld>".parse().unwrap())
        .reply_to("Yuin <yuin@domain.tld>".parse().unwrap())
        .to("Hei <hei@domain.tld>".parse().unwrap())
        .subject("Happy new async year")
        .header(ContentType::TEXT_PLAIN)
        .body(String::from("Be happy with async!"))
        .unwrap();

    let creds = Credentials::new("smtp_username".to_owned(), "smtp_password".to_owned());

    // Open a remote connection to gmail
    let mailer: AsyncSmtpTransport<Tokio1Executor> =
        AsyncSmtpTransport::<Tokio1Executor>::relay("smtp.gmail.com")
            .unwrap()
            .credentials(creds)
            .build();

    // Send the email
    match mailer.send(email).await {
        Ok(_) => println!("Email sent successfully!"),
        Err(e) => panic!("Could not send email: {e:?}"),
    }
}