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.
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.