The part that I'm not understanding, and at this point I will put it in my todo list to sign up for a free forwarding account in Zoho to try it out, is how it is possible for the Zoho mail server to forward emails to Gmail without the messages being flagged as SPF fail, since your Zoho server is not authorized to dispatch emails from the sender domain when they have SPF records.
In other words, this is a real example from an email that just got flagged on my Gmail from apple.com, as part of being forwarded:
Received: from relay5-d.mail.gandi.net (relay5-d.mail.gandi.net. [217.70.183.197] by mx.google.com ...
Received-SPF: softfail (google.com: domain of transitioning [email protected] does not designate 217.70.183.197 as
permitted sender) client-ip=217.70.183.197;
Received: from nwk-txn-msbadger0502.apple.com (nwk-txn-msbadger0502.apple.com [17.151.1.69]) by spool.mail.gandi.net ...
Date: Mon, 21 Jan 2019 03:18:35 +0000 (GMT)
From: Apple <[email protected]>
Reply-To: [email protected]
1) The email is being sent from [email protected]
2) The email is received by the Gandi mail server from Apple mail server
3) The email is received by the Google mail server from Gandi mail server
Step 3 is where the forwarding happens but, since the Gandi mail server is not supposed to originate emails from apple.com (according to the SPF records on the apple.com domain), then it gets marked as SPF fail, and the Google mail server tells why (ip address of the Gandi mail server is not allowed to generate mail from [email protected]).
I don't understand how that cannot happen with Zoho, and I'll study it asap.
Delivered-To: (ME)@gmail.com
Received: by 2002:a9d:650d:0:0:0:0:0...
Received: from sender-of-f72.zoho.com (sender-of-f72.zoho.com. [135.84.80.227]) by mx.google.com...
Received-SPF: pass (google.com: domain of (ME)+aml_=(MY DOMAIN)=gmail.com@(MY DOMAIN) designates 135.84.80.227 as permitted sender) client-ip=135.84.80.227;
Authentication-Results: mx.google.com;
dkim=pass [email protected] header.s=id0517 header.b=S5Hlyy1T;
spf=pass (google.com: domain of (ME)+aml_=(MY DOMAIN)=gmail.com@(MY DOMAIN) designates 135.84.80.227 as permitted sender) smtp.mailfrom="(ME)+aml_=(MY DOMAIN)=gmail.com@(MY DOMAIN)";
dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=id.apple.com
Received-SPF: pass (zoho.com: domain of email.apple.com designates 17.151.1.66 as permitted sender) client-ip=17.151.1.66; [email protected]; helo=nwk-txn-msbadger0405.apple.com;
Authentication-Results: mx.zohomail.com;
dkim=pass;
spf=pass (zoho.com: domain of email.apple.com designates 17.151.1.66 as permitted sender) [email protected];
dmarc=pass(p=reject dis=none) header.from=id.apple.com
Received: from nwk-txn-msbadger0405.apple.com (nwk-txn-msbadger0405.apple.com [17.151.1.66]) by mx.zohomail.com...
From: Apple <[email protected]>Looking at the headers, it seems clear that the Zoho mail server does something different when forwarding the message: it also sets some parts of the headers (like smtp.mailfrom) to instruct the recipient mail server that the mail is sent from your email address, and not really from @id.apple.com. Google then validates not @id.apple.com, but (ME)+aml_=(MY DOMAIN)=gmail.com@(MY DOMAIN), which makes it work.
I will ask Gandi support.
Edit: after doing some research, it appears Zoho is implementing SRS (http://www.openspf.org/SRS) as the diagrams in the web page show, whereas Gandi is not. I just learned a new thing!
Thanks again.
The emails get accepted into the Zoho mailbox, then forwarded to Gmail. If Gmail rejects the forwarded email (due to DMARC policy) then I get the failure notification, not the original sender (as the failure is that Zoho was unable to forward the email to Gmail, not that Zoho didn't accept the original email from the sender), and that email will be retrieved by Gmail when it next does it's POP3 check.