diff --git a/app/src/main/java/eu/faircode/email/AdapterContact.java b/app/src/main/java/eu/faircode/email/AdapterContact.java
index ad0eb19482..503118d84f 100644
--- a/app/src/main/java/eu/faircode/email/AdapterContact.java
+++ b/app/src/main/java/eu/faircode/email/AdapterContact.java
@@ -48,6 +48,7 @@ import androidx.recyclerview.widget.RecyclerView;
 import java.text.NumberFormat;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 
 public class AdapterContact extends RecyclerView.Adapter<AdapterContact.ViewHolder> {
     private Fragment parentFragment;
@@ -322,9 +323,9 @@ public class AdapterContact extends RecyclerView.Adapter<AdapterContact.ViewHold
             items = all;
         else {
             items = new ArrayList<>();
-            String query = search.toLowerCase().trim();
+            String query = search.toLowerCase(Locale.ROOT).trim();
             for (TupleContactEx contact : contacts)
-                if (contact.email.toLowerCase().contains(query) ||
+                if (contact.email.toLowerCase(Locale.ROOT).contains(query) ||
                         (contact.name != null && contact.name.toLowerCase().contains(query)))
                     items.add(contact);
         }
diff --git a/app/src/main/java/eu/faircode/email/AdapterMessage.java b/app/src/main/java/eu/faircode/email/AdapterMessage.java
index d02874dc5d..04be385108 100644
--- a/app/src/main/java/eu/faircode/email/AdapterMessage.java
+++ b/app/src/main/java/eu/faircode/email/AdapterMessage.java
@@ -137,6 +137,7 @@ import java.util.Collections;
 import java.util.Date;
 import java.util.HashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Objects;
 
@@ -1851,7 +1852,7 @@ public class AdapterMessage extends RecyclerView.Adapter<AdapterMessage.ViewHold
         private void onNotifyContact(final TupleMessageEx message) {
             final NotificationManager nm = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
             final InternetAddress from = (InternetAddress) message.from[0];
-            final String channelId = "notification." + from.getAddress().toLowerCase();
+            final String channelId = "notification." + from.getAddress().toLowerCase(Locale.ROOT);
 
             PopupMenuLifecycle popupMenu = new PopupMenuLifecycle(context, powner, ibAddContact);
             NotificationChannel channel = nm.getNotificationChannel(channelId);
@@ -3666,7 +3667,7 @@ public class AdapterMessage extends RecyclerView.Adapter<AdapterMessage.ViewHold
                 boolean changed = false;
                 builder.clearQuery();
                 for (String key : uri.getQueryParameterNames())
-                    if (PARANOID_QUERY.contains(key.toLowerCase()))
+                    if (PARANOID_QUERY.contains(key.toLowerCase(Locale.ROOT)))
                         changed = true;
                     else if (!TextUtils.isEmpty(key))
                         for (String value : uri.getQueryParameters(key)) {
diff --git a/app/src/main/java/eu/faircode/email/BoundaryCallbackMessages.java b/app/src/main/java/eu/faircode/email/BoundaryCallbackMessages.java
index 766f54bec6..5ece84feca 100644
--- a/app/src/main/java/eu/faircode/email/BoundaryCallbackMessages.java
+++ b/app/src/main/java/eu/faircode/email/BoundaryCallbackMessages.java
@@ -42,6 +42,7 @@ import java.util.Arrays;
 import java.util.Calendar;
 import java.util.Date;
 import java.util.List;
+import java.util.Locale;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 
@@ -172,7 +173,7 @@ public class BoundaryCallbackMessages extends PagedList.BoundaryCallback<TupleMe
         try {
             db.beginTransaction();
 
-            String find = (TextUtils.isEmpty(query) ? null : query.toLowerCase());
+            String find = (TextUtils.isEmpty(query) ? null : query.toLowerCase(Locale.ROOT));
             for (int i = state.index; i < state.messages.size() && found < pageSize && !state.destroyed; i++) {
                 state.index = i + 1;
 
@@ -204,17 +205,17 @@ public class BoundaryCallbackMessages extends PagedList.BoundaryCallback<TupleMe
                         for (Address address : addresses) {
                             String email = ((InternetAddress) address).getAddress();
                             String name = ((InternetAddress) address).getPersonal();
-                            if (email != null && email.toLowerCase().contains(find) ||
-                                    name != null && name.toLowerCase().contains(find))
+                            if (email != null && email.toLowerCase(Locale.ROOT).contains(find) ||
+                                    name != null && name.toLowerCase(Locale.ROOT).contains(find))
                                 match = true;
                         }
 
                         if (!match && message.subject != null)
-                            match = message.subject.toLowerCase().contains(find);
+                            match = message.subject.toLowerCase(Locale.ROOT).contains(find);
 
                         if (!match && message.keywords != null && message.keywords.length > 0)
                             for (String keyword : message.keywords)
-                                if (keyword.toLowerCase().contains(find)) {
+                                if (keyword.toLowerCase(Locale.ROOT).contains(find)) {
                                     match = true;
                                     break;
                                 }
@@ -222,7 +223,7 @@ public class BoundaryCallbackMessages extends PagedList.BoundaryCallback<TupleMe
                         if (!match && message.content) {
                             try {
                                 String body = Helper.readText(message.getFile(context));
-                                match = body.toLowerCase().contains(find);
+                                match = body.toLowerCase(Locale.ROOT).contains(find);
                             } catch (IOException ex) {
                                 Log.e(ex);
                             }
diff --git a/app/src/main/java/eu/faircode/email/Core.java b/app/src/main/java/eu/faircode/email/Core.java
index 7d2c963d2f..5d5bfc396b 100644
--- a/app/src/main/java/eu/faircode/email/Core.java
+++ b/app/src/main/java/eu/faircode/email/Core.java
@@ -80,6 +80,7 @@ import java.util.Comparator;
 import java.util.Date;
 import java.util.HashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Properties;
@@ -1345,7 +1346,7 @@ class Core {
             boolean sync_kept = prefs.getBoolean("sync_kept", true);
             boolean delete_unseen = prefs.getBoolean("delete_unseen", false);
 
-            if (account.host.toLowerCase().contains("imap.zoho")) {
+            if (account.host.toLowerCase(Locale.ROOT).contains("imap.zoho")) {
                 sync_unseen = false;
                 sync_flagged = false;
             }
@@ -2266,7 +2267,8 @@ class Core {
             if (Build.VERSION.SDK_INT > Build.VERSION_CODES.O &&
                     message.notifying == 0 && message.from != null && message.from.length > 0) {
                 InternetAddress from = (InternetAddress) message.from[0];
-                NotificationChannel channel = nm.getNotificationChannel("notification." + from.getAddress().toLowerCase());
+                NotificationChannel channel = nm.getNotificationChannel(
+                        "notification." + from.getAddress().toLowerCase(Locale.ROOT));
                 if (channel != null && channel.getImportance() == NotificationManager.IMPORTANCE_NONE)
                     continue;
             }
@@ -2481,7 +2483,8 @@ class Core {
 
                 if (message.from != null && message.from.length > 0) {
                     InternetAddress from = (InternetAddress) message.from[0];
-                    channel = nm.getNotificationChannel("notification." + from.getAddress().toLowerCase());
+                    channel = nm.getNotificationChannel(
+                            "notification." + from.getAddress().toLowerCase(Locale.ROOT));
                 }
 
                 if (channel == null)
diff --git a/app/src/main/java/eu/faircode/email/EmailProvider.java b/app/src/main/java/eu/faircode/email/EmailProvider.java
index d38b1a13c3..3cab3b4639 100644
--- a/app/src/main/java/eu/faircode/email/EmailProvider.java
+++ b/app/src/main/java/eu/faircode/email/EmailProvider.java
@@ -160,12 +160,12 @@ public class EmailProvider {
     static EmailProvider fromDomain(Context context, String domain, Discover discover) throws IOException {
         List<EmailProvider> providers = loadProfiles(context);
         for (EmailProvider provider : providers)
-            if (provider.domain != null && provider.domain.contains(domain.toLowerCase())) {
+            if (provider.domain != null && provider.domain.contains(domain.toLowerCase(Locale.ROOT))) {
                 Log.i("Provider from domain=" + domain);
                 return provider;
             }
 
-        EmailProvider autoconfig = _fromDomain(context, domain.toLowerCase(), discover);
+        EmailProvider autoconfig = _fromDomain(context, domain.toLowerCase(Locale.ROOT), discover);
 
         // Always prefer built-in profiles
         // - ISPDB is not always correct
diff --git a/app/src/main/java/eu/faircode/email/EntityFolder.java b/app/src/main/java/eu/faircode/email/EntityFolder.java
index 485f4114a2..b858d09111 100644
--- a/app/src/main/java/eu/faircode/email/EntityFolder.java
+++ b/app/src/main/java/eu/faircode/email/EntityFolder.java
@@ -303,7 +303,7 @@ public class EntityFolder extends EntityOrder implements Serializable {
 
     static String guessType(String fullName) {
         for (String guess : GUESS_FOLDER_TYPE.keySet())
-            if (fullName.toLowerCase().contains(guess))
+            if (fullName.toLowerCase(Locale.ROOT).contains(guess))
                 return GUESS_FOLDER_TYPE.get(guess);
         return null;
     }
diff --git a/app/src/main/java/eu/faircode/email/EntityRule.java b/app/src/main/java/eu/faircode/email/EntityRule.java
index 48f47c4b7a..ece01d3bb5 100644
--- a/app/src/main/java/eu/faircode/email/EntityRule.java
+++ b/app/src/main/java/eu/faircode/email/EntityRule.java
@@ -44,6 +44,7 @@ import java.util.Calendar;
 import java.util.Date;
 import java.util.Enumeration;
 import java.util.List;
+import java.util.Locale;
 import java.util.regex.Pattern;
 
 import javax.mail.Address;
@@ -250,7 +251,8 @@ public class EntityRule {
                 Pattern pattern = Pattern.compile(needle, Pattern.DOTALL);
                 matched = pattern.matcher(haystack).matches();
             } else
-                matched = haystack.toLowerCase().contains(needle.trim().toLowerCase());
+                matched = haystack.toLowerCase(Locale.ROOT)
+                        .contains(needle.trim().toLowerCase(Locale.ROOT));
 
         Log.i("Rule=" + name + " matched=" + matched +
                 " needle=" + needle + " haystack=" + haystack + " regex=" + regex);
diff --git a/app/src/main/java/eu/faircode/email/FragmentCompose.java b/app/src/main/java/eu/faircode/email/FragmentCompose.java
index 3608ebfba9..f5d493b659 100644
--- a/app/src/main/java/eu/faircode/email/FragmentCompose.java
+++ b/app/src/main/java/eu/faircode/email/FragmentCompose.java
@@ -1142,7 +1142,7 @@ public class FragmentCompose extends FragmentBase {
 
                 String[] tos = new String[ato.length];
                 for (int i = 0; i < ato.length; i++)
-                    tos[i] = ato[i].getAddress().toLowerCase();
+                    tos[i] = ato[i].getAddress().toLowerCase(Locale.ROOT);
 
                 Intent intent = new Intent(OpenPgpApi.ACTION_GET_KEY_IDS);
                 intent.putExtra(OpenPgpApi.EXTRA_USER_IDS, tos);
@@ -1865,7 +1865,8 @@ public class FragmentCompose extends FragmentBase {
 
             String extension = Helper.getExtension(attachment.name);
             if (extension != null)
-                attachment.type = MimeTypeMap.getSingleton().getMimeTypeFromExtension(extension.toLowerCase());
+                attachment.type = MimeTypeMap.getSingleton()
+                        .getMimeTypeFromExtension(extension.toLowerCase(Locale.ROOT));
             if (attachment.type == null)
                 attachment.type = "application/octet-stream";
             attachment.disposition = (image ? Part.INLINE : Part.ATTACHMENT);
diff --git a/app/src/main/java/eu/faircode/email/Helper.java b/app/src/main/java/eu/faircode/email/Helper.java
index 0e51c486f3..b9f7f608d3 100644
--- a/app/src/main/java/eu/faircode/email/Helper.java
+++ b/app/src/main/java/eu/faircode/email/Helper.java
@@ -92,7 +92,6 @@ import java.util.Date;
 import java.util.List;
 import java.util.Locale;
 import java.util.Objects;
-import java.util.Set;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ThreadFactory;
@@ -393,7 +392,7 @@ public class Helper {
 
     static String localizeFolderType(Context context, String type) {
         int resid = context.getResources().getIdentifier(
-                "title_folder_" + type.toLowerCase(),
+                "title_folder_" + type.toLowerCase(Locale.ROOT),
                 "string",
                 context.getPackageName());
         return (resid > 0 ? context.getString(resid) : type);
diff --git a/app/src/main/java/eu/faircode/email/HtmlHelper.java b/app/src/main/java/eu/faircode/email/HtmlHelper.java
index f83dfc9d33..55388c963d 100644
--- a/app/src/main/java/eu/faircode/email/HtmlHelper.java
+++ b/app/src/main/java/eu/faircode/email/HtmlHelper.java
@@ -71,6 +71,7 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
+import java.util.Locale;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.regex.Matcher;
@@ -154,10 +155,14 @@ public class HtmlHelper {
                 for (String param : params) {
                     String[] kv = param.split(":");
                     if (kv.length == 2)
-                        switch (kv[0].trim().toLowerCase()) {
+                        switch (kv[0].trim().toLowerCase(Locale.ROOT)) {
                             case "color":
                                 sb.append(param).append(";");
                                 break;
+
+                            case "background":
+                            case "background-color":
+                                break;
                         }
                 }
 
@@ -849,8 +854,8 @@ public class HtmlHelper {
             if (params.length > 0) {
                 List<String> viewport = new ArrayList<>();
                 for (String param : params)
-                    if (!param.toLowerCase().contains("maximum-scale") &&
-                            !param.toLowerCase().contains("user-scalable"))
+                    if (!param.toLowerCase(Locale.ROOT).contains("maximum-scale") &&
+                            !param.toLowerCase(Locale.ROOT).contains("user-scalable"))
                         viewport.add(param.trim());
 
                 if (viewport.size() == 0)
diff --git a/app/src/main/java/eu/faircode/email/MailService.java b/app/src/main/java/eu/faircode/email/MailService.java
index 7fd5dd838e..69b4a27e7d 100644
--- a/app/src/main/java/eu/faircode/email/MailService.java
+++ b/app/src/main/java/eu/faircode/email/MailService.java
@@ -18,6 +18,7 @@ import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.LinkedHashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Properties;
 import java.util.concurrent.ExecutorService;
@@ -65,7 +66,7 @@ public class MailService implements AutoCloseable {
         properties.put("mail.event.scope", "folder");
         properties.put("mail.event.executor", executor);
 
-        String checkserveridentity = Boolean.toString(!insecure).toLowerCase();
+        String checkserveridentity = Boolean.toString(!insecure).toLowerCase(Locale.ROOT);
 
         if ("pop3".equals(protocol) || "pop3s".equals(protocol)) {
             this.debug = true;
diff --git a/app/src/main/java/eu/faircode/email/MessageHelper.java b/app/src/main/java/eu/faircode/email/MessageHelper.java
index 3af7df9e84..bf5e092ba6 100644
--- a/app/src/main/java/eu/faircode/email/MessageHelper.java
+++ b/app/src/main/java/eu/faircode/email/MessageHelper.java
@@ -50,6 +50,7 @@ import java.util.Date;
 import java.util.Enumeration;
 import java.util.HashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Properties;
@@ -1136,7 +1137,7 @@ public class MessageHelper {
 
                         try {
                             ContentType ct = new ContentType(cpart.getContentType());
-                            if ("application/pgp-encrypted".equals(ct.getBaseType().toLowerCase())) {
+                            if ("application/pgp-encrypted".equals(ct.getBaseType().toLowerCase(Locale.ROOT))) {
                                 pgp = true;
                                 continue;
                             }
@@ -1157,7 +1158,7 @@ public class MessageHelper {
                 try {
                     disposition = part.getDisposition();
                     if (disposition != null)
-                        disposition = disposition.toLowerCase();
+                        disposition = disposition.toLowerCase(Locale.ROOT);
                 } catch (MessagingException ex) {
                     Log.w(ex);
                     parts.warnings.add(Helper.formatThrowable(ex, false));
@@ -1213,7 +1214,7 @@ public class MessageHelper {
 
                     apart.attachment = new EntityAttachment();
                     apart.attachment.name = apart.filename;
-                    apart.attachment.type = ct.getBaseType().toLowerCase();
+                    apart.attachment.type = ct.getBaseType().toLowerCase(Locale.ROOT);
                     apart.attachment.disposition = apart.disposition;
                     apart.attachment.size = (long) apart.part.getSize();
                     apart.attachment.cid = (cid == null || cid.length == 0 ? null : MimeUtility.unfold(cid[0]));
@@ -1227,9 +1228,10 @@ public class MessageHelper {
                     if (!apart.pgp) {
                         String extension = Helper.getExtension(apart.attachment.name);
                         if (extension != null &&
-                                ("pdf".equals(extension.toLowerCase()) ||
+                                ("pdf".equals(extension.toLowerCase(Locale.ROOT)) ||
                                         "application/octet-stream".equals(apart.attachment.type))) {
-                            String type = MimeTypeMap.getSingleton().getMimeTypeFromExtension(extension.toLowerCase());
+                            String type = MimeTypeMap.getSingleton()
+                                    .getMimeTypeFromExtension(extension.toLowerCase(Locale.ROOT));
                             if (type != null) {
                                 if (!type.equals(apart.attachment.type))
                                     Log.w("Guessing file=" + apart.attachment.name + " type=" + type);