Added SMTP DSN

app/src/main/java/eu/faircode/email/EmailService.java

@@ -263,6 +263,11 @@ public class EmailService implements AutoCloseable {
         properties.put("mail.mime.allowutf8", Boolean.toString(value));
     }
 
+    // https://tools.ietf.org/html/rfc3461
+    void setDsnNotify(String what) {
+        properties.put("mail." + protocol + ".dsn.notify", what);
+    }
+
     void setListener(StoreListener listener) {
         this.listener = listener;
     }

app/src/main/java/eu/faircode/email/ServiceSend.java

@@ -584,6 +584,12 @@ public class ServiceSend extends ServiceBase implements SharedPreferences.OnShar
             iservice.setUseIp(ident.use_ip, ident.ehlo);
             iservice.setUnicode(ident.unicode);
 
+            if (message.receipt_request != null && message.receipt_request) {
+                int receipt_type = prefs.getInt("receipt_type", 2);
+                if (receipt_type == 1 || receipt_type == 2) // Delivery receipt
+                    iservice.setDsnNotify("SUCCESS,FAILURE,DELAY");
+            }
+
             // Connect transport
             db.identity().setIdentityState(ident.id, "connecting");
             iservice.connect(ident);