Email supports end to end encryption between any two servers. You set each server up with a list of supported protocols and denied protocols.
If either side can't agree on the encryption algorithm, then the email does not go through.
If both sides do agree, which is almost always the case if both sides are using industry standard encryption methods, then the email is end to end encrypted.
On top of that, we have PGP encryption which allows the user to encrypt the email from everybody including the email provider. Only the recipient email address and possibly the subject line would be readable to the email providers.
As for the IP, if protonmail removes the sender's origin IP then it's removed and that's it.
Email is one of our best methods of international communication available today.