> Or is Google sending the message as `nso@MyCoolDomain.com via nso@gmail.com` (and exposing your Gmail address in replies)? Yeah the gmail account shows up in

> Or is Google sending the message as `nso@MyCoolDomain.com via nso@gmail.com` (and exposing your Gmail address in replies)? Yeah the gmail account shows up in the headers as Return-path. Google don't DKIM sign it (there's a X-Google-DKIM-Signature header, but that's obviously ignored), so I think it'll still trigger DMARC if you have that setup. However, their SMTP servers should still reject other people sending on that address' behalf if they attempt to. It'll just bounce and send you an undeliverable mail along the lines of 'You no longer have access to blah@customdomain'