diff -up evolution-3.10.4/modules/text-highlight/e-mail-formatter-text-highlight.c.large-message-render-hung evolution-3.10.4/modules/text-highlight/e-mail-formatter-text-highlight.c
--- evolution-3.10.4/modules/text-highlight/e-mail-formatter-text-highlight.c.large-message-render-hung	2013-12-07 15:14:12.000000000 +0100
+++ evolution-3.10.4/modules/text-highlight/e-mail-formatter-text-highlight.c	2014-09-12 08:24:26.418217142 +0200
@@ -41,6 +41,15 @@ typedef EMailFormatterExtensionClass EMa
 typedef EExtension EMailFormatterTextHighlightLoader;
 typedef EExtensionClass EMailFormatterTextHighlightLoaderClass;
 
+typedef struct _TextHighlightClosure TextHighlightClosure;
+
+struct _TextHighlightClosure {
+	CamelStream *read_stream;
+	CamelStream *output_stream;
+	GCancellable *cancellable;
+	GError *error;
+};
+
 GType e_mail_formatter_text_highlight_get_type (void);
 
 G_DEFINE_DYNAMIC_TYPE (
@@ -110,6 +119,103 @@ get_syntax (EMailPart *part,
 	return syntax;
 }
 
+static gpointer
+text_hightlight_read_data_thread (gpointer user_data)
+{
+	TextHighlightClosure *closure = user_data;
+	gchar buffer[10240];
+
+	g_return_val_if_fail (closure != NULL, NULL);
+
+	while (!camel_stream_eos (closure->read_stream) &&
+	       !g_cancellable_set_error_if_cancelled (closure->cancellable, &closure->error)) {
+		gssize read;
+		gsize wrote = 0;
+
+		read = camel_stream_read (closure->read_stream, buffer, 10240, closure->cancellable, &closure->error);
+		if (read < 0 || closure->error)
+			break;
+
+		wrote = camel_stream_write (closure->output_stream, buffer, read, closure->cancellable, &closure->error);
+		if (wrote == -1 || (gssize) wrote != read || closure->error)
+			break;
+	}
+
+	return NULL;
+}
+
+static gboolean
+text_highlight_feed_data (CamelStream *output_stream,
+                          CamelDataWrapper *data_wrapper,
+                          gint pipe_stdin,
+                          gint pipe_stdout,
+                          GCancellable *cancellable,
+                          GError **error)
+{
+	TextHighlightClosure closure;
+	CamelContentType *content_type;
+	CamelStream *write_stream;
+	gboolean success = TRUE;
+	GThread *thread;
+
+	closure.read_stream = camel_stream_fs_new_with_fd (pipe_stdout);
+	closure.output_stream = output_stream;
+	closure.cancellable = cancellable;
+	closure.error = NULL;
+
+	write_stream = camel_stream_fs_new_with_fd (pipe_stdin);
+
+	thread = g_thread_new (NULL, text_hightlight_read_data_thread, &closure);
+
+	content_type = camel_data_wrapper_get_mime_type_field (data_wrapper);
+	if (content_type) {
+		const gchar *charset = camel_content_type_param (content_type, "charset");
+
+		/* Convert to UTF-8 charset, if needed, which the 'highlight' expects;
+		   it can cope with non-UTF-8 letters, thus no need for a content UTF-8-validation */
+		if (charset && g_ascii_strcasecmp (charset, "utf-8") != 0) {
+			CamelMimeFilter *filter;
+
+			filter = camel_mime_filter_charset_new (charset, "UTF-8");
+			if (filter != NULL) {
+				CamelStream *filtered = camel_stream_filter_new (write_stream);
+
+				if (filtered) {
+					camel_stream_filter_add (CAMEL_STREAM_FILTER (filtered), filter);
+					g_object_unref (write_stream);
+					write_stream = filtered;
+				}
+
+				g_object_unref (filter);
+			}
+		}
+	}
+
+	if (camel_data_wrapper_decode_to_stream_sync (data_wrapper, write_stream, cancellable, error) < 0) {
+		g_cancellable_cancel (cancellable);
+		success = FALSE;
+	} else {
+		/* Close the stream, thus the highlight knows no more data will come */
+		g_clear_object (&write_stream);
+	}
+
+	g_thread_join (thread);
+
+	g_clear_object (&closure.read_stream);
+	g_clear_object (&write_stream);
+
+	if (closure.error) {
+		if (error && !*error)
+			g_propagate_error (error, closure.error);
+		else
+			g_clear_error (&closure.error);
+
+		return FALSE;
+	}
+
+	return success;
+}
+
 static gboolean
 emfe_text_highlight_format (EMailFormatterExtension *extension,
                             EMailFormatter *formatter,
@@ -213,35 +319,27 @@ emfe_text_highlight_format (EMailFormatt
 			&pid, &pipe_stdin, &pipe_stdout, NULL, NULL);
 
 		if (success) {
-			CamelStream *read;
-			CamelStream *write;
-			CamelStream *utf8;
-			GByteArray *ba;
-			gchar *tmp;
-
-			write = camel_stream_fs_new_with_fd (pipe_stdin);
-			read = camel_stream_fs_new_with_fd (pipe_stdout);
-
-			/* Decode the content of mime part to the 'utf8' stream */
-			utf8 = camel_stream_mem_new ();
-			camel_data_wrapper_decode_to_stream_sync (
-				dw, utf8, cancellable, NULL);
-
-			/* Convert the binary data do someting displayable */
-			ba = camel_stream_mem_get_byte_array (CAMEL_STREAM_MEM (utf8));
-			tmp = e_util_utf8_data_make_valid ((gchar *) ba->data, ba->len);
-
-			/* Send the sanitized data to the highlighter */
-			camel_stream_write_string (write, tmp, cancellable, NULL);
-			g_free (tmp);
-			g_object_unref (utf8);
-			g_object_unref (write);
+			GError *local_error = NULL;
 
-			g_spawn_close_pid (pid);
+			success = text_highlight_feed_data (
+				stream, dw,
+				pipe_stdin, pipe_stdout,
+				cancellable, &local_error);
+
+			if (g_error_matches (
+				local_error, G_IO_ERROR,
+				G_IO_ERROR_CANCELLED)) {
+				/* Do nothing. */
+
+			} else if (local_error != NULL) {
+				g_warning (
+					"%s: %s", G_STRFUNC,
+					local_error->message);
+			}
+
+			g_clear_error (&local_error);
 
-			g_seekable_seek (G_SEEKABLE (read), 0, G_SEEK_SET, cancellable, NULL);
-			camel_stream_write_to_stream (read, stream, cancellable, NULL);
-			g_object_unref (read);
+			g_spawn_close_pid (pid);
 		} else {
 			/* We can't call e_mail_formatter_format_as on text/plain,
 			 * because text-highlight is registered as an handler for