This isn’t anything new though.
E-Mails may get forwarded several times between MTAs, whereas every MTA can (and must) see (at least) the headers of the message.
That is one of the reasons for deploying (and using) end to end encryption: end to end encrypted email bodies cannot be read in transit by forwarding MTAs.
The padlock that google shows only indicates that the transit between the last MTA outside of google and google has been encrypted; this does not mean however that the transit MTAs not read (and possibly even modify) the email.
You can witness this behavior with several free web mail providers who append an advertising signature to emails even if they are sent via a mail client. That is possible because the MTA has access to the whole of the message when processing it. A malicious MTA in this chain (you might call it a proxy from a functional point of view if it is the first hop of outbound and last hop of inbound mails) can easily copy all emails.
That being said, one would have to have control over an MTA within the delivery chain of all emails, but there are several ways how to do that.
Also: there is no way to tell wether an email has been copied in-transit while being processed by an MTA, other than the other answer is suggesting.