Thanks, really appreciate you taking the time to extract those headers, very helpful. Looking at the headers, it seems clear that the Zoho mail server does som

Thanks, really appreciate you taking the time to extract those headers, very helpful. 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 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.