In my understanding (please correct if applicable),
- The e-mails sent through a secure server are encrypted in the route between me as client and the mail server, whether this is a local or remote one.
- The server-to-server transit across the web is generally insecure.
- At the other end, the recipient' server may again have a secure connection with his/her client.
So there is a potential gap of security in the middle.
Is there a way to ascertain whether an email has sailed the web, server-to-server, securely? This question applies to all parts of a mail (header, content, attachment).