More new messages icon

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

@@ -2783,7 +2783,9 @@ class Core {
             // Build notification
             NotificationCompat.Builder builder =
                     new NotificationCompat.Builder(context, "notification")
-                            .setSmallIcon(R.drawable.baseline_mail_white_24)
+                            .setSmallIcon(messages.size() > 1
+                                    ? R.drawable.baseline_mail_more_white_24
+                                    : R.drawable.baseline_mail_white_24)
                             .setContentTitle(title)
                             .setContentIntent(piUnified)
                             .setNumber(messages.size())

app/src/main/res/drawable/baseline_mail_more_24.xml

@@ -0,0 +1,19 @@
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="24dp"
+    android:height="24dp"
+    android:tint="?attr/colorControlNormal"
+    android:viewportWidth="24.0"
+    android:viewportHeight="24.0">
+    <group
+        android:translateX="2.0"
+        android:translateY="-2.0">
+        <path
+            android:fillColor="@android:color/white"
+            android:pathData="M20,4L4,4c-1.1,0 -1.99,0.9 -1.99,2L2,18c0,1.1 0.9,2 2,2h16c1.1,0 2,-0.9 2,-2L22,6c0,-1.1 -0.9,-2 -2,-2zM20,8l-8,5 -8,-5L4,6l8,5 8,-5v2z" />
+    </group>
+    <group>
+        <path
+            android:fillColor="@android:color/white"
+            android:pathData="M2,6L0,6v5h0.01L0,20c0,1.1 0.9,2 2,2h18v-2L2,20L2,6z" />
+    </group>
+</vector>