DKIM, spam blocking and .forward'ed mail
My organisation owns more than one domain. Our primary domain has Zoho mail. The other domains are merely forwarded (for both web and e-mail) by our domain name registrar (hexonet.net) so that everything redirects to our main, primary domain.
User mail for our other domains is forwarded,
user@my.example.biz ->
user@my-zoho-domain.example.net for instance.
Hexonet's mail servers are forwarding.ispapi.net or xdns-smtp2.forwarding.ispapi.net according to these headers:
Authentication-Results: mx.zohomail.com;
spf=pass (zoho.com: domain of forwarding.ispapi.net designates 84.200.110.122 as permitted sender) smtp.mailfrom=SRS0=tmjT=AI=my.example.biz=user@forwarding.ispapi.net
Received: from xdns-smtp2.forwarding.ispapi.net (xdns-smtp2.forwarding.ispapi.net [84.200.110.122]) by mx.zohomail.com
with SMTP id 1504818059894280.23004094217936; Thu, 7 Sep 2017 14:00:59 -0700 (PDT)
Received: by xdns-smtp2.forwarding.ispapi.net (Postfix, from userid 1000)
id A6F1A60E76; Thu, 7 Sep 2017 21:00:58 +0000 (UTC)
I'd enabled the inbound spam quarantine for both SPF and DKIM, only to find a huge quantity of forwarded user mail sidelined as failing the DKIM tests. I've since turned both off.
I presume, from the headers, that my issue is with DKIM. Should I whitelist all of my upstream providers (so ispapi.net, forwarding.ispapi.net and the like) or should I just leave DKIM checks on inbound mail shut off?