lib/PublicInbox/MDA.pm | 3 +++

diff --git a/lib/PublicInbox/MDA.pm b/lib/PublicInbox/MDA.pm
index e2b2f9143083c0b1979d283fda7d2b985096e9a9..d2ce79eb210fe62b52160ab5ad0b08178e98d4a6 100644
--- a/lib/PublicInbox/MDA.pm
+++ b/lib/PublicInbox/MDA.pm
@@ -65,6 +65,9 @@ 	my $pa = "<$dst->{-primary_address}>";
 	$simple->header_set("List-Id", $pa);
 	$simple->header_set("List-Post", $pa);
 
+	# prevent training loops
+	$simple->header_set('Delivered-To');
+
 	my $url = $dst->{url};
 	if (defined $url) {
 		$simple->header_set("List-Archive", "<$url>");