Switch to BC on unsupported protocol

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

@@ -73,6 +73,7 @@ import javax.mail.MessagingException;
 import javax.net.SocketFactory;
 import javax.net.ssl.HostnameVerifier;
 import javax.net.ssl.HttpsURLConnection;
+import javax.net.ssl.SSLHandshakeException;
 import javax.net.ssl.SSLSession;
 import javax.net.ssl.SSLSocket;
 import javax.net.ssl.SSLSocketFactory;
@@ -504,6 +505,17 @@ public class ConnectionHelper {
         return false;
     }
 
+    static boolean isUnsupportedProtocol(Throwable ex) {
+        while (ex != null) {
+            if (ex instanceof SSLHandshakeException &&
+                    ex.getMessage() != null &&
+                    ex.getMessage().contains("UNSUPPORTED_PROTOCOL"))
+                return true;
+            ex = ex.getCause();
+        }
+        return false;
+    }
+
     static boolean isAborted(Throwable ex) {
         while (ex != null) {
             String msg = ex.getMessage();

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

@@ -742,6 +742,24 @@ public class EmailService implements AutoCloseable {
                     ((ErrnoException) ex.getCause().getCause()).errno == OsConstants.EACCES)
                 throw new SecurityException("EACCES Please check 'Restrict data usage' in the Android app settings", ex);
 
+            if (!ssl_harden && ConnectionHelper.isUnsupportedProtocol(ex)) {
+                EntityLog.log(context, EntityLog.Type.Network, "Unsuported protocol");
+                try {
+                    this.insecure = true;
+                    factory = new SSLSocketFactoryService(context,
+                            host, port, true, false,
+                            false, false, false, false,
+                            false,
+                            true, true,
+                            factory.key, factory.chain, factory.trustedFingerprint);
+                    properties.put("mail." + protocol + ".ssl.socketFactory", factory);
+                    _connect(main, port, require_id, user, factory);
+                    return;
+                } catch (GeneralSecurityException ex1) {
+                    Log.e(ex1);
+                }
+                return;
+            } else {
                 boolean ioError = false;
                 Throwable ce = ex;
                 while (ce != null) {
@@ -818,6 +836,7 @@ public class EmailService implements AutoCloseable {
                         throw new MessagingException(ex1.getMessage(), ex1);
                     }
                 }
+            }
 
             throw ex;
         }
@@ -1111,6 +1130,8 @@ public class EmailService implements AutoCloseable {
         private boolean secure;
         private boolean ssl_harden;
         private boolean ssl_harden_strict;
+        private PrivateKey key;
+        private X509Certificate[] chain;
         private String trustedFingerprint;
         private SSLSocketFactory factory;
         private X509Certificate certificate;
@@ -1125,6 +1146,8 @@ public class EmailService implements AutoCloseable {
             this.secure = !insecure;
             this.ssl_harden = ssl_harden;
             this.ssl_harden_strict = ssl_harden_strict;
+            this.key = key;
+            this.chain = chain;
             this.trustedFingerprint = fingerprint;
 
             TrustManager[] tms = SSLHelper.getTrustManagers(