diff --git a/app/build.gradle b/app/build.gradle
index 3922d2e7da..53f1e78058 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -117,8 +117,10 @@ dependencies {
     def dnsjava_version = "2.1.8"
     def openpgp_version = "12.0"
     def requery_version = "3.26.0"
-    def junit_version = "5.3.1"
+    def junit_version = "4.12"
     def assertj_version = "3.11.1"
+    def mockito_version = "2.24.0"
+    def powermock_version = "2.0.0"
 
     // https://developer.android.com/jetpack/docs/release-notes
 
@@ -181,10 +183,17 @@ dependencies {
     // git clone https://android.googlesource.com/platform/frameworks/opt/colorpicker
     implementation project(path: ':colorpicker')
 
-    // https://junit.org/junit5/
-    testImplementation "org.junit.jupiter:junit-jupiter-api:$junit_version"
-    testRuntimeOnly "org.junit.jupiter:junit-jupiter-engine:$junit_version"
+    // https://junit.org/
+    testImplementation "junit:junit:$junit_version"
 
     // https://joel-costigliola.github.io/assertj/
     testImplementation "org.assertj:assertj-core:$assertj_version"
+
+    // https://site.mockito.org/
+    testImplementation "org.mockito:mockito-core:$mockito_version"
+
+    // https://github.com/powermock/powermock
+    testImplementation "org.powermock:powermock-core:$powermock_version"
+    testImplementation "org.powermock:powermock-api-mockito2:$powermock_version"
+    testImplementation "org.powermock:powermock-module-junit4:$powermock_version"
 }
diff --git a/app/src/main/java/eu/faircode/email/HtmlHelper.java b/app/src/main/java/eu/faircode/email/HtmlHelper.java
index f07e6e9fd3..413383d1b9 100644
--- a/app/src/main/java/eu/faircode/email/HtmlHelper.java
+++ b/app/src/main/java/eu/faircode/email/HtmlHelper.java
@@ -130,8 +130,8 @@ public class HtmlHelper {
         StringBuilder sb = new StringBuilder();
         int end = 0;
         while (matcher.find()) {
-            sb.append(html(text.substring(end, matcher.start())));
-            String ref = html(matcher.group());
+            sb.append(Html.escapeHtml(text.substring(end, matcher.start())));
+            String ref = Html.escapeHtml(matcher.group());
             sb.append(String.format("<a href=\"%s\">%s</a>", ref, ref));
             end = matcher.end();
         }
@@ -139,10 +139,6 @@ public class HtmlHelper {
         return sb.toString();
     }
 
-    private static String html(String plain) {
-        return plain.replace("&", "&amp;").replace("<", "&lt;");
-    }
-
     static Drawable decodeImage(String source, Context context, long id, boolean show) {
         int px = Helper.dp2pixels(context, 48);
 
diff --git a/app/src/test/java/eu/faircode/email/HtmlHelperTest.java b/app/src/test/java/eu/faircode/email/HtmlHelperTest.java
index 9ca5787e46..613afc2e44 100644
--- a/app/src/test/java/eu/faircode/email/HtmlHelperTest.java
+++ b/app/src/test/java/eu/faircode/email/HtmlHelperTest.java
@@ -1,13 +1,39 @@
 package eu.faircode.email;
 
-import org.junit.jupiter.api.Test;
+import android.text.Html;
 
-import static org.assertj.core.api.Assertions.assertThat;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.invocation.InvocationOnMock;
+import org.mockito.stubbing.Answer;
+import org.powermock.core.classloader.annotations.PrepareForTest;
+import org.powermock.modules.junit4.PowerMockRunner;
 
-class HtmlHelperTest {
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.powermock.api.mockito.PowerMockito.mockStatic;
+import static org.powermock.api.mockito.PowerMockito.when;
+
+@PrepareForTest(Html.class)
+@RunWith(PowerMockRunner.class)
+public class HtmlHelperTest {
+
+    @Before
+    public void setUp() {
+        mockStatic(Html.class);
+        when(Html.escapeHtml(any(CharSequence.class))).thenAnswer(new Answer<Object>() {
+            @Override
+            public Object answer(InvocationOnMock invocation) {
+                return invocation.getArgument(0).toString()
+                        .replace("&", "&amp;")
+                        .replace("<", "&lt;");
+            }
+        });
+    }
 
     @Test
-    void autolink() {
+    public void autolink() {
 
         testAutolink(
                 "To visit http://www.example.org, go to http://www.example.org.",