// Build a simple multipart messageletmessage = MessageBuilder::new()        .from(("John Doe","john@example.com"))        .to(vec![            ("Jane Doe","jane@example.com"),            ("James Smith","james@test.com"),        ])        .subject("Hi!")        .html_body("<h1>Hello, world!</h1>")        .text_body("Hello world!");// Connect to the SMTP submissions port, upgrade to TLS and    // authenticate using the provided credentials.SmtpClientBuilder::new("smtp.gmail.com",587)        .implicit_tls(false)        .credentials(("john","p4ssw0rd"))        .connect()        .await.unwrap()        .send(message)        .await.unwrap();

// Build a simple text message with a single attachmentletmessage = MessageBuilder::new()        .from(("John Doe","john@example.com"))        .to("jane@example.com")        .subject("Howdy!")        .text_body("These pretzels are making me thirsty.")        .attachment("image/png","pretzels.png", [1,2,3,4].as_ref());// Sign an e-mail message using RSA-SHA256letpk_rsa = RsaKey::<Sha256>::from_rsa_pem(TEST_KEY).unwrap();letsigner = DkimSigner::from_key(pk_rsa)        .domain("example.com")        .selector("default")        .headers(["From","To","Subject"])        .expiration(60*60*7);// Number of seconds before this signature expires (optional)    // Connect to an SMTP relay server over TLS.    // Signs each message with the configured DKIM signer.SmtpClientBuilder::new("smtp.gmail.com",465)        .connect()        .await.unwrap()        .send_signed(message,&signer)        .await.unwrap();

 $ cargo test --all-features

 $ cargo +nightly miri test --all-features