diff --git a/core/postfix/conf/master.cf b/core/postfix/conf/master.cf
index b43095ee..e45a8ccf 100644
--- a/core/postfix/conf/master.cf
+++ b/core/postfix/conf/master.cf
@@ -12,6 +12,7 @@ smtp      inet  n       -       n       -       -       smtpd
   -o cleanup_service_name=outclean
 outclean  unix n       -       n       -       0       cleanup
   -o header_checks=pcre:/etc/postfix/outclean_header_filter.cf
+  -o nested_header_checks=
 
 # Internal postfix services
 pickup    unix  n       -       n       60      1       pickup
diff --git a/towncrier/newsfragments/1660.bugfix b/towncrier/newsfragments/1660.bugfix
new file mode 100644
index 00000000..a90fb099
--- /dev/null
+++ b/towncrier/newsfragments/1660.bugfix
@@ -0,0 +1 @@
+Don't replace nested headers (typically in attached emails)