[PATCH] mda: reject multiple Message-IDs up front

[Eric Wong <e@80x24.org>]

While ssoma now documents it uses the first Message-ID, they
are confusing and could be a sign of a broken mail software,
and broken mail software is often a sign of spam...

ref: http://public-inbox.org/meta/20160421221128.4910-1-e@80x24.org/
---
 lib/PublicInbox/MDA.pm | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/PublicInbox/MDA.pm b/lib/PublicInbox/MDA.pm
index 003bac6..26681c2 100644
--- a/lib/PublicInbox/MDA.pm
+++ b/lib/PublicInbox/MDA.pm
@@ -23,7 +23,9 @@ sub __drop_plus {
 sub precheck {
 	my ($klass, $filter, $address) = @_;
 	my Email::Simple $simple = $filter->simple;
-	my $mid = $simple->header("Message-ID");
+	my @mid = $simple->header('Message-ID');
+	return 0 if scalar(@mid) != 1;
+	my $mid = $mid[0];
 	return 0 if (length($mid) > MAX_MID_SIZE);
 	return 0 unless usable_str(length('<m@h>'), $mid) && $mid =~ /\@/;
 	return 0 unless usable_str(length('u@h'), $filter->from);
-- 
EW