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I'm not so sure about that; I use a self-signed cert for port 25 TLS, and I just sent from google to my domain, and didn't see a warning.

What's your source for that?

That doesn't bring a lot of extra security though, because there's no name verification. I can get a valid letsencrypt cert on anyrandomdomain.com, and if I can hijack your MX and point at it, it's "valid".

What? How can you get a cert for a domain you don't control?

I don't need to control your domain. If I control my own domain, which could be any throwaway domain I just purchased, I can get an SSL certificate on it.

And if I can point your MX records there, via hijack or any other means, then I have a valid SSL certificate for receiving your email.