Check for providers with a proprietary protocol

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

@@ -80,6 +80,15 @@ public class EmailProvider {
     private static final int SCAN_TIMEOUT = 5 * 1000; // milliseconds
     private static final int ISPDB_TIMEOUT = 15 * 1000; // milliseconds
 
+    private static final List<String> PROPRIETARY = Collections.unmodifiableList(Arrays.asList(
+            "protonmail.ch",
+            "protonmail.com",
+            "tutanota.com",
+            "tutanota.de",
+            "tutamail.com", // tutanota
+            "tuta.io", // tutanota
+            "keemailme" // tutanota
+    ));
     private static final ExecutorService executor =
             Helper.getBackgroundExecutor(0, "provider");
 
@@ -194,6 +203,9 @@ public class EmailProvider {
         if (at < 0)
             email = "someone@" + domain;
 
+        if (PROPRIETARY.contains(domain))
+            throw new IllegalArgumentException(context.getString(R.string.title_no_standard));
+
         List<EmailProvider> providers = loadProfiles(context);
         for (EmailProvider provider : providers)
             if (provider.domain != null)

app/src/main/res/values/strings.xml

@@ -513,6 +513,7 @@
     <string name="title_no_inbox">Inbox not found</string>
     <string name="title_no_primary_drafts">No primary account or no drafts folder</string>
     <string name="title_no_identities">Sending emails requires at least one identity</string>
+    <string name="title_no_standard">This provider uses a proprietary email protocol and therefore it is not possible to use third part email clients</string>
     <string name="title_no_idle">This provider does not support push messages. This will delay reception of new messages and increase battery usage.</string>
     <string name="title_no_utf8">This provider does not support UTF-8</string>
     <string name="title_no_sync">Synchronization errors since %1$s</string>