diff --git a/src/calibre/utils/smtp.py b/src/calibre/utils/smtp.py
index 9fef6b8e6d..3e880a5347 100644
--- a/src/calibre/utils/smtp.py
+++ b/src/calibre/utils/smtp.py
@@ -248,6 +248,7 @@ def main(args=sys.argv):
     except:
         if outbox is not None:
             outbox.add(msg)
+            outbox.close()
             print 'Delivery failed. Message saved to', opts.outbox
         raise
     return 0