Use opportunistic TLS in SMTP connections

If SSL is disabled, the SMTP ClientSecurity of the lettre crate
defaults to None, that is, an insecure connection. This is changed to
Opportunistic, which uses TLS if available. If TLS is not available,
the insecure connection is used (i.e., this change is backward
compatible).

src/mail.rs

@@ -18,21 +18,21 @@ use chrono::NaiveDateTime;
 fn mailer() -> SmtpTransport {
     let host = CONFIG.smtp_host().unwrap();
 
-    let client_security = if CONFIG.smtp_ssl() {
     let tls = TlsConnector::builder()
         .min_protocol_version(Some(Protocol::Tlsv11))
         .build()
         .unwrap();
 
-        let params = ClientTlsParameters::new(host.clone(), tls);
+    let tls_params = ClientTlsParameters::new(host.clone(), tls);
 
+    let client_security = if CONFIG.smtp_ssl() {
         if CONFIG.smtp_explicit_tls() {
-            ClientSecurity::Wrapper(params)
+            ClientSecurity::Wrapper(tls_params)
         } else {
-            ClientSecurity::Required(params)
+            ClientSecurity::Required(tls_params)
         }
     } else {
-        ClientSecurity::None
+        ClientSecurity::Opportunistic(tls_params)
     };
 
     use std::time::Duration;