diff -ura notmuch-0.27/notmuch-show.c notmuch-0.27-patched/notmuch-show.c
--- notmuch-0.27/notmuch-show.c	2018-06-13 03:42:34.000000000 +0200
+++ notmuch-0.27-patched/notmuch-show.c	2018-07-28 10:25:25.358502880 +0200
@@ -271,7 +271,10 @@
 {
     GMimeContentType *content_type = g_mime_object_get_content_type (GMIME_OBJECT (part));
     GMimeStream *stream_filter = NULL;
+    GMimeStream *null_stream = NULL;
+    GMimeStream *null_stream_filter = NULL;
     GMimeFilter *crlf_filter = NULL;
+    GMimeFilter *windows_filter = NULL;
     GMimeDataWrapper *wrapper;
     const char *charset;
 
@@ -282,13 +285,27 @@
     if (stream_out == NULL)
 	return;
 
+    charset = g_mime_object_get_content_type_parameter (part, "charset");
+    wrapper = g_mime_part_get_content_object (GMIME_PART (part));
+    if (wrapper && charset) {
+	/* Check for mislabeled Windows encoding */
+	null_stream = g_mime_stream_null_new ();
+	null_stream_filter = g_mime_stream_filter_new (null_stream);
+	windows_filter = g_mime_filter_windows_new (charset);
+	g_mime_stream_filter_add(GMIME_STREAM_FILTER (null_stream_filter),
+				 windows_filter);
+	g_mime_data_wrapper_write_to_stream (wrapper, null_stream_filter);
+	charset = g_mime_filter_windows_real_charset(
+	    (GMimeFilterWindows *) windows_filter);
+	g_object_unref (windows_filter);
+    }
+
     stream_filter = g_mime_stream_filter_new (stream_out);
     crlf_filter = g_mime_filter_crlf_new (false, false);
     g_mime_stream_filter_add(GMIME_STREAM_FILTER (stream_filter),
 			     crlf_filter);
     g_object_unref (crlf_filter);
 
-    charset = g_mime_object_get_content_type_parameter (part, "charset");
     if (charset) {
 	GMimeFilter *charset_filter;
 	charset_filter = g_mime_filter_charset_new (charset, "UTF-8");
@@ -313,9 +330,12 @@
 	}
     }
 
-    wrapper = g_mime_part_get_content_object (GMIME_PART (part));
     if (wrapper && stream_filter)
 	g_mime_data_wrapper_write_to_stream (wrapper, stream_filter);
+    if (null_stream_filter)
+	g_object_unref (null_stream_filter);
+    if (null_stream)
+	g_object_unref (null_stream);
     if (stream_filter)
 	g_object_unref(stream_filter);
 }