diff --git a/app/src/main/java/eu/faircode/email/EntityFolder.java b/app/src/main/java/eu/faircode/email/EntityFolder.java
index e57daea554..20ba7dba04 100644
--- a/app/src/main/java/eu/faircode/email/EntityFolder.java
+++ b/app/src/main/java/eu/faircode/email/EntityFolder.java
@@ -390,7 +390,7 @@ public class EntityFolder extends EntityOrder implements Serializable {
                 return null;
 
             if (attr.startsWith("\\")) {
-                int index = SYSTEM_FOLDER_ATTR.indexOf(attr.substring(1).toLowerCase());
+                int index = SYSTEM_FOLDER_ATTR.indexOf(attr.substring(1).toLowerCase(Locale.ROOT));
                 if (index >= 0)
                     return SYSTEM_FOLDER_TYPE.get(index);
             }
diff --git a/app/src/main/java/eu/faircode/email/EntityIdentity.java b/app/src/main/java/eu/faircode/email/EntityIdentity.java
index 00791c22cb..6b9a57708c 100644
--- a/app/src/main/java/eu/faircode/email/EntityIdentity.java
+++ b/app/src/main/java/eu/faircode/email/EntityIdentity.java
@@ -31,6 +31,7 @@ import androidx.room.PrimaryKey;
 import org.json.JSONException;
 import org.json.JSONObject;
 
+import java.util.Locale;
 import java.util.Objects;
 import java.util.regex.Pattern;
 
@@ -167,7 +168,7 @@ public class EntityIdentity {
             if (user.equalsIgnoreCase(cemail[0]))
                 return true;
         } else {
-            String input = (sender_extra_regex.contains("@") ? other.toLowerCase() : cother[0]);
+            String input = (sender_extra_regex.contains("@") ? other.toLowerCase(Locale.ROOT) : cother[0]);
             if (Pattern.matches(sender_extra_regex, input))
                 return true;
         }
diff --git a/app/src/main/java/eu/faircode/email/FragmentCompose.java b/app/src/main/java/eu/faircode/email/FragmentCompose.java
index fc3b5b0bc8..b173e57a75 100644
--- a/app/src/main/java/eu/faircode/email/FragmentCompose.java
+++ b/app/src/main/java/eu/faircode/email/FragmentCompose.java
@@ -2057,7 +2057,7 @@ public class FragmentCompose extends FragmentBase {
                     List<String> emails = new ArrayList<>();
                     for (int i = 0; i < recipients.size(); i++) {
                         InternetAddress recipient = (InternetAddress) recipients.get(i);
-                        String email = recipient.getAddress().toLowerCase();
+                        String email = recipient.getAddress().toLowerCase(Locale.ROOT);
                         if (!emails.contains(email))
                             emails.add(email);
                     }
@@ -2677,7 +2677,7 @@ public class FragmentCompose extends FragmentBase {
                                     Intent intent = new Intent(OpenPgpApi.ACTION_GET_KEY);
                                     intent.putExtra(OpenPgpApi.EXTRA_KEY_ID, pgpSignKeyId);
                                     intent.putExtra(OpenPgpApi.EXTRA_MINIMIZE, true);
-                                    intent.putExtra(OpenPgpApi.EXTRA_MINIMIZE_USER_ID, identity.email.toLowerCase());
+                                    intent.putExtra(OpenPgpApi.EXTRA_MINIMIZE_USER_ID, identity.email.toLowerCase(Locale.ROOT));
                                     intent.putExtra(OpenPgpApi.EXTRA_REQUEST_ASCII_ARMOR, true);
                                     intent.putExtra(BuildConfig.APPLICATION_ID, largs);
                                     return intent;
@@ -2697,7 +2697,7 @@ public class FragmentCompose extends FragmentBase {
                                 Intent intent = new Intent(OpenPgpApi.ACTION_GET_KEY);
                                 intent.putExtra(OpenPgpApi.EXTRA_KEY_ID, pgpSignKeyId);
                                 intent.putExtra(OpenPgpApi.EXTRA_MINIMIZE, true);
-                                intent.putExtra(OpenPgpApi.EXTRA_MINIMIZE_USER_ID, identity.email.toLowerCase());
+                                intent.putExtra(OpenPgpApi.EXTRA_MINIMIZE_USER_ID, identity.email.toLowerCase(Locale.ROOT));
                                 intent.putExtra(OpenPgpApi.EXTRA_REQUEST_ASCII_ARMOR, true);
                                 intent.putExtra(BuildConfig.APPLICATION_ID, largs);
                                 return intent;
@@ -4969,7 +4969,7 @@ public class FragmentCompose extends FragmentBase {
                                         String[] userIds = new String[recipients.size()];
                                         for (int i = 0; i < recipients.size(); i++) {
                                             InternetAddress recipient = (InternetAddress) recipients.get(i);
-                                            userIds[i] = recipient.getAddress().toLowerCase();
+                                            userIds[i] = recipient.getAddress().toLowerCase(Locale.ROOT);
                                         }
 
                                         Intent intent = new Intent(OpenPgpApi.ACTION_GET_KEY_IDS);
diff --git a/app/src/main/java/eu/faircode/email/FragmentOAuth.java b/app/src/main/java/eu/faircode/email/FragmentOAuth.java
index abc1493228..d74b41f8eb 100644
--- a/app/src/main/java/eu/faircode/email/FragmentOAuth.java
+++ b/app/src/main/java/eu/faircode/email/FragmentOAuth.java
@@ -75,6 +75,7 @@ import java.util.ArrayList;
 import java.util.Date;
 import java.util.HashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 
 import javax.mail.AuthenticationFailedException;
@@ -688,7 +689,7 @@ public class FragmentOAuth extends FragmentBase {
             if (ex instanceof AuthenticationFailedException)
                 tvOfficeAuthHint.setVisibility(View.VISIBLE);
 
-            String e = etEmail.getText().toString().toLowerCase();
+            String e = etEmail.getText().toString().toLowerCase(Locale.ROOT);
             if (e.contains("@outlook") ||
                     e.contains("@live") ||
                     e.contains("@hotmail") ||
diff --git a/app/src/main/java/eu/faircode/email/HtmlHelper.java b/app/src/main/java/eu/faircode/email/HtmlHelper.java
index b796e91a00..4f9e92060f 100644
--- a/app/src/main/java/eu/faircode/email/HtmlHelper.java
+++ b/app/src/main/java/eu/faircode/email/HtmlHelper.java
@@ -1739,7 +1739,7 @@ public class HtmlHelper {
                 for (int i = 0; i < param.length; i++) {
                     String[] kv = param[i].split("=");
                     if (kv.length == 2) {
-                        switch (kv[0].replace(" ", "").toLowerCase()) {
+                        switch (kv[0].replace(" ", "").toLowerCase(Locale.ROOT)) {
                             case "user-scalable":
                                 kv[1] = "yes";
                                 param[i] = TextUtils.join("=", kv);
diff --git a/app/src/main/java/eu/faircode/email/MessageHelper.java b/app/src/main/java/eu/faircode/email/MessageHelper.java
index f328195a34..e9188bf6a8 100644
--- a/app/src/main/java/eu/faircode/email/MessageHelper.java
+++ b/app/src/main/java/eu/faircode/email/MessageHelper.java
@@ -2826,9 +2826,9 @@ public class MessageHelper {
         String email1 = ((InternetAddress) a1).getAddress();
         String email2 = ((InternetAddress) a2).getAddress();
         if (email1 != null)
-            email1 = email1.toLowerCase();
+            email1 = email1.toLowerCase(Locale.ROOT);
         if (email2 != null)
-            email2 = email2.toLowerCase();
+            email2 = email2.toLowerCase(Locale.ROOT);
         return Objects.equals(email1, email2);
     }