diff --git a/app/src/main/java/eu/faircode/email/EntityFolder.java b/app/src/main/java/eu/faircode/email/EntityFolder.java
index 291ef2e6f6..81f7043e4e 100644
--- a/app/src/main/java/eu/faircode/email/EntityFolder.java
+++ b/app/src/main/java/eu/faircode/email/EntityFolder.java
@@ -110,7 +110,7 @@ public class EntityFolder implements Serializable {
     static final String USER = "User";
 
     // https://www.iana.org/assignments/imap-mailbox-name-attributes/imap-mailbox-name-attributes.xhtml
-    private static final List<String> SYSTEM_FOLDER_ATTR = Arrays.asList(
+    private static final List<String> SYSTEM_FOLDER_ATTR = Collections.unmodifiableList(Arrays.asList(
             "All",
             "Archive",
             "Drafts",
@@ -119,8 +119,8 @@ public class EntityFolder implements Serializable {
             "Sent",
             "Important",
             "Flagged"
-    );
-    private static final List<String> SYSTEM_FOLDER_TYPE = Arrays.asList(
+    ));
+    private static final List<String> SYSTEM_FOLDER_TYPE = Collections.unmodifiableList(Arrays.asList(
             ARCHIVE, // All
             ARCHIVE,
             DRAFTS,
@@ -129,9 +129,9 @@ public class EntityFolder implements Serializable {
             SENT,
             SYSTEM, // Important
             SYSTEM // Flagged
-    ); // MUST match SYSTEM_FOLDER_ATTR
+    )); // MUST match SYSTEM_FOLDER_ATTR
 
-    static final List<String> FOLDER_SORT_ORDER = Arrays.asList(
+    static final List<String> FOLDER_SORT_ORDER = Collections.unmodifiableList(Arrays.asList(
             INBOX,
             OUTBOX,
             DRAFTS,
@@ -141,28 +141,28 @@ public class EntityFolder implements Serializable {
             JUNK,
             SYSTEM,
             USER
-    );
+    ));
 
     static final int DEFAULT_INIT = 14; // days
     static final int DEFAULT_SYNC = 7; // days
     static final int DEFAULT_KEEP = 30; // days
 
-    static final List<String> SYSTEM_FOLDER_SYNC = Arrays.asList(
+    static final List<String> SYSTEM_FOLDER_SYNC = Collections.unmodifiableList(Arrays.asList(
             INBOX,
             DRAFTS,
             SENT,
             ARCHIVE,
             TRASH,
             JUNK
-    );
-    static final List<Boolean> SYSTEM_FOLDER_DOWNLOAD = Arrays.asList(
+    ));
+    static final List<Boolean> SYSTEM_FOLDER_DOWNLOAD = Collections.unmodifiableList(Arrays.asList(
             true, // inbox
             true, // drafts
             false, // sent
             false, // archive
             false, // trash
             false // junk
-    ); // MUST match SYSTEM_FOLDER_SYNC
+    )); // MUST match SYSTEM_FOLDER_SYNC
 
     public EntityFolder() {
     }
diff --git a/app/src/main/java/eu/faircode/email/FragmentMessages.java b/app/src/main/java/eu/faircode/email/FragmentMessages.java
index c0dbae89de..38d30d33ec 100644
--- a/app/src/main/java/eu/faircode/email/FragmentMessages.java
+++ b/app/src/main/java/eu/faircode/email/FragmentMessages.java
@@ -175,7 +175,7 @@ public class FragmentMessages extends FragmentBase {
     private static final int UNDO_TIMEOUT = 5000; // milliseconds
     private static final int SWIPE_DISABLE_SELECT_DURATION = 1500; // milliseconds
 
-    private static final List<String> DUPLICATE_ORDER = Arrays.asList(
+    private static final List<String> DUPLICATE_ORDER = Collections.unmodifiableList(Arrays.asList(
             EntityFolder.INBOX,
             EntityFolder.OUTBOX,
             EntityFolder.DRAFTS,
@@ -185,7 +185,7 @@ public class FragmentMessages extends FragmentBase {
             EntityFolder.SYSTEM,
             EntityFolder.USER,
             EntityFolder.ARCHIVE
-    );
+    ));
 
     @Override
     public void onCreate(Bundle savedInstanceState) {
diff --git a/app/src/main/java/eu/faircode/email/HtmlHelper.java b/app/src/main/java/eu/faircode/email/HtmlHelper.java
index ecb79ea037..1527ff2155 100644
--- a/app/src/main/java/eu/faircode/email/HtmlHelper.java
+++ b/app/src/main/java/eu/faircode/email/HtmlHelper.java
@@ -52,6 +52,7 @@ import java.io.InputStream;
 import java.io.OutputStream;
 import java.net.URL;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.regex.Matcher;
 
@@ -67,8 +68,10 @@ public class HtmlHelper {
     static final int PREVIEW_SIZE = 250;
 
     private static final int TRACKING_PIXEL_SURFACE = 25;
-    private static final List<String> heads = Arrays.asList("h1", "h2", "h3", "h4", "h5", "h6", "p", "table", "ol", "ul", "br", "hr");
-    private static final List<String> tails = Arrays.asList("h1", "h2", "h3", "h4", "h5", "h6", "p", "ol", "ul", "li");
+    private static final List<String> heads = Collections.unmodifiableList(Arrays.asList(
+            "h1", "h2", "h3", "h4", "h5", "h6", "p", "ol", "ul", "table", "br", "hr"));
+    private static final List<String> tails = Collections.unmodifiableList(Arrays.asList(
+            "h1", "h2", "h3", "h4", "h5", "h6", "p", "ol", "ul", "li"));
 
     static String removeTracking(Context context, String html) {
         SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(context);