--- AddFooter.java      2017-12-07 17:39:02.887926900 +0900
+++ AddFooter.java.fix  2017-12-07 17:42:32.017173600 +0900
@@ -94,11 +94,19 @@
         if (part.isMimeType("multipart/mixed")
                 || part.isMimeType("multipart/related")) {
             MimeMultipart multipart = (MimeMultipart) part.getContent();
-            return attachFooterToFirstPart(multipart);
+            boolean added = attachFooterToFirstPart(multipart);
+            if (added) {
+               part.setContent(multipart);
+            }
+            return added;

         } else if (part.isMimeType("multipart/alternative")) {
             MimeMultipart multipart = (MimeMultipart) part.getContent();
-            return attachFooterToAllSubparts(multipart);
+            boolean added = attachFooterToAllSubparts(multipart);
+            if (added) {
+               part.setContent(multipart);
+            }
+            return added;
         }
         //Give up... we won't attach the footer to this MimePart
         return false;