Gateway services could exist, but I am not sure it would be worth it.
Perhaps a simpler way would be for mail servers to have a SAFENetwork back end for mailbox files. This way, multiple mail server heads could read/write to the same mailbox, using a common storage. This could be achieved through a simple drive mount.
You could have more complex solutions where a mail server is aware of your personal (read: stored to your own personal SAFENetwork account) mailbox directly and watches the outbox for new mails to relay. You could have the reverse as some sort of shared appendage data for incoming mail too. The incoming mail could also be sent internally to SAFENetwork.
Just spit balling really. Tbh though, email is used less and less and people have adopted new IM apps with little resistance (telegram, WhatsApp, Facebook chat, Skype, etc). Considering that any relay exposes the same issues as the current email system, I’m not sure it buys us much either.
Remember, you can run your own personal mail server right now if you want to. Just point your DNS MX records to your static IP with mail server on it and you are good to go. The main issue is with spam, the lack of static IPs and the fact that most people don’t want the hassle. It is fairly distributed as is though, even if it is too open to abuse.