If Gmail is rejecting your custom domain, you haven’t setup your SPF, or DKIM records etc for you custom domain that authorizes proton to be a valid email sender.
E.g when Gmai receives an email from a custom domain it will look up records to confirm the mail server sending it is valid.
Checkout https://proton.me/support/anti-spoofing-custom-domain or search proton SPF DKIM for guides.
You wouldnt have issues sending from a proton domain as they already have these in place for you.







Great all the way through - enjoy the ride.