alt Hacker NewsI self-host my email, in part because I wanted to enforce encryption whenever I send. My server does verify the certificate of the target MX, whether it uses MTA-STS or not. Delivery fails if the certificate is invalid, and also fails if the destination (or a man in the middle) downgrades or strips STARTTLS.
In eight years, I’ve seen zero servers that refused STARTTLS, and only two invalid certificates. In both cases, the certificates were from a valid CA and weren’t expired, but the hostname was wrong. Both were due to outdated DNS records, one of which was fixed as soon as I reported it, and the other resolved itself after a few months.
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I am self-hosted: minimal smtp server, no TLS, only direct and basic delivery of emails. I don't even pay the DNS mob, namely I use IP literals (mailbox@[x.x.x.x] and more mailbox@[ipv6:...], see smtp RFC, yeah the email guys were smart enough not to make their protocol hard dependent on DNS neither on any crypto).
I do not trust the CA system. If I ever crypt the content of an email, it would be because a guy I saw gave me its public key in some way (QR code?) face to face. For really serious stuff, I would not use smtp anyway and with some modified-standard-cipher pre-shared key anyways.
> My server does verify the certificate of the target MX
But how do you know if the target MX is correct, and not controlled by an attacker? You can't really know for sure unless the receiver has secured their domain name with DNSSEC or published an MTA-STS policy.