We spent a lot of time thinking about this before we started building Let's Encrypt. In the case of the Web we didn't think there was a way to make self-signed certificates workable for anything because older browsers will always error on them. In the case of mailservers, self-signed certificates work for enabling TLS, and that's great, but you're extremely vulnerable to MITM attacks. So this project offers a way to prevent the MITMs, if you want it.
If there aren't major technical obstacles we might be willing to take pull requests for STARTTLS Everywhere that allow mailservers to announce self-signing policies, but it hasn't been a priority thus far because LE certs are so easy to get and are slightly more authenticated.
If there aren't major technical obstacles we might be willing to take pull requests for STARTTLS Everywhere that allow mailservers to announce self-signing policies, but it hasn't been a priority thus far because LE certs are so easy to get and are slightly more authenticated.