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 noreply@apple.com 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 <noreply@apple.com>
Reply-To: noreply@apple.com
1) The email is being sent from noreply@apple.com
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 noreply@apple.com).
I don't understand how that cannot happen with Zoho, and I'll study it asap.