mid: filter out 'y', 'n', and email addresses from references()

Looking at git@vger history, several emails had broken
References/In-Reply-To pointing to <y>, <n> and email
addresses as Message-IDs in References and In-Reply-To
headers.

This was causing too many unrelated messages to be linked
together in the same thread.

1 files changed, 4 insertions, 0 deletions

diff --git a/t/mid.t b/t/mid.t
index 8c307c82..69a8a708 100644
--- a/t/mid.t
+++ b/t/mid.t
@@ -36,6 +36,10 @@ is(mid_escape('foo%!@(bar)'), 'foo%25!@(bar)');
         $mime->header_set('Message-ID', "<hello\tworld>");
         is_deeply(mids($mime->header_obj), ['helloworld'],
                 'drop \t in Message-ID');
+
+        $mime->header_set('To', 'u@example.com');
+        $mime->header_set('References', '<hello> <world> <n> <u@example.com>');
+        is_deeply(references($mime->header_obj), [qw(hello world)]);
 }
 
 done_testing();