Header has unknown for IP address

[Sophie Loewenthal]

Hi,

I don’t believe my Postfix install is doing the PTR lookups, or it broke in 
some way.  

I have this enabled: 
# postconf |grep smtpd_peername_lookup
smtpd_peername_lookup = yes
 
and my DNS can resolve the name:
# host 168.100.1.3
3.1.100.168.in-addr.arpa domain name pointer camomile.cloud9.net.

But the headers give unknown:
Received: from camomile.cloud9.net (unknown [168.100.1.3])
        by mx10.example.com (Postfix) with ESMTPS id BA9143C
        for <u...@example.com>; Wed, 27 Jun 2018 15:48:44 +0000 (UTC)


How could I debug/enable this? What was missed in the config.

Many thanks, S.