Spam filters isn’t the same as defederating. As far as I know outside of cert issues (like DKIM to prevent spoofing) nobody would prevent you from sending an email to any domain that uses SMTP. And if you allowlist emails from that domain you’ll receive it.
This is not the same as Gmail saying “we won’t allow emails to and from proton” or vice versa.
Spam filters isn’t the same as defederating. As far as I know outside of cert issues (like DKIM to prevent spoofing) nobody would prevent you from sending an email to any domain that uses SMTP. And if you allowlist emails from that domain you’ll receive it.
This is not the same as Gmail saying “we won’t allow emails to and from proton” or vice versa.
But MANY email services block entire domains. Not “to” I’ll give you but from.