Protip: When setting up Mastodon to use smtp-relay.gmail.com make sure you add both the servers ipv4 and ipv6 addresses to the allowed senders. Otherwise you just get a very generic EOF error from the mailer process. Doh! 😭
I had to write a short ruby script to test in order to actually identify the error coming from back from the SMTP server.
Mail is working now and people can actually sign up properly. Success! ✔️