lib/PublicInbox/NNTP.pm |  7 +++++++
 t/nntpd.t               | 20 ++++++++++++++++++++

diff --git a/lib/PublicInbox/NNTP.pm b/lib/PublicInbox/NNTP.pm
index 1d07220de1ec977fb30ba296c93a989f8ddafe11..d409e7828f29e2d6d75f8d8309fee027d3665c69 100644
--- a/lib/PublicInbox/NNTP.pm
+++ b/lib/PublicInbox/NNTP.pm
@@ -519,6 +519,13 @@ sub _header ($) {
 	my $hdr = $_[0]->header_obj->as_string;
 	utf8::encode($hdr);
 	$hdr =~ s/(?<!\r)\n/\r\n/sg;
+
+	# for leafnode compatibility, we need to ensure Message-ID headers
+	# are only a single line.  We can't subclass Email::Simple::Header
+	# and override _default_fold_at in here, either; since that won't
+	# affect messages already in the archive.
+	$hdr =~ s/^(Message-ID:)[ \t]*\r\n[ \t]+([^\r]+)\r\n/$1 $2\r\n/igsm;
+
 	$hdr
 }
 
diff --git a/t/nntpd.t b/t/nntpd.t
index c73cc122eaa009618fd6bab0df357fe5b23e6173..cce21eeb620d09bfc9c783993ce5de743b94d9fa 100644
--- a/t/nntpd.t
+++ b/t/nntpd.t
@@ -231,6 +231,26 @@ 		my $t1 = time;
 		ok($date >= $t0, 'valid date after start');
 		ok($date <= $t1, 'valid date before stop');
 	}
+	if ('leafnode interop') {
+		my $for_leafnode = PublicInbox::MIME->new(<<"");
+From: longheader\@example.com
+To: $addr
+Subject: none
+Date: Fri, 02 Oct 1993 00:00:00 +0000
+
+		my $long_hdr = 'for-leafnode-'.('y'x200).'@example.com';
+		$for_leafnode->header_set('Message-ID', "<$long_hdr>");
+		$im->add($for_leafnode);
+		$im->done;
+		if ($version == 1) {
+			my $s = PublicInbox::SearchIdx->new($mainrepo, 1);
+			$s->index_sync;
+		}
+		my $hdr = $n->head("<$long_hdr>");
+		my $expect = qr/\AMessage-ID: /i . qr/\Q<$long_hdr>\E/;
+		ok(scalar(grep(/$expect/, @$hdr)), 'Message-ID not folded');
+		ok(scalar(grep(/^Path:/, @$hdr)), 'Path: header found');
+	}
 
 	# pipelined requests:
 	{