Update to tokio 1.0 (#529)

examples/tokio1_smtp_tls.rs

@@ -0,0 +1,36 @@
+// This line is only to make it compile from lettre's examples folder,
+// since it uses Rust 2018 crate renaming to import tokio.
+// Won't be needed in user's code.
+use tokio1_crate as tokio;
+
+use lettre::{
+    transport::smtp::authentication::Credentials, AsyncSmtpTransport, Message, Tokio1Connector,
+    Tokio1Transport,
+};
+
+#[tokio::main]
+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")
+        .body(String::from("Be happy with async!"))
+        .unwrap();
+
+    let creds = Credentials::new("smtp_username".to_string(), "smtp_password".to_string());
+
+    // Open a remote connection to gmail
+    let mailer = AsyncSmtpTransport::<Tokio1Connector>::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),
+    }
+}