Sure, but it's security by obscurity and relying an the receiving and sending servers to maintain that cache long enough at both ends.
The problem is that the DNS contains an MX record that says where to deliver email for a domain. So when a sending server wants to send, it looks that up. Then it looks up the received name as an A/AAAA record to find the IP address to connect to.
So we're already relying on DNS, which can be poisoned.
The "fix" for DNS poisoning is not DNS-over-TLS/HTTPS because that basic problem isn't solved.
If we secure DNS against tampering/poisoning, then the same infrastructure can deliver public keys and other information safely as well.
Is maintaining a cache difficult? Disk space is cheap and databases are reliable in 2024.
I'll mention that the MTA-STS policy includes a list of permitted MX, so a poisoned MX record would be detected. But you are still correct about "first email to a domain" concerns; MTA-STS does not offer protection when the policy isn't in the cache.
There's some obvious improvements we could consider, like a community list of domains known to support MTA-STS, and mail servers pre-warming their caches. That gets us something closer to the HSTS preload list, which protects the first connection to a domain. MTA-STS is a reasonable foundation for future work.
It's also really worth remembering that the current best practices for email are unencrypted connections, skipping TLS cert verification, and sometimes DANE. A solution that protects all but the first email is a big step forward.
The problem is that the DNS contains an MX record that says where to deliver email for a domain. So when a sending server wants to send, it looks that up. Then it looks up the received name as an A/AAAA record to find the IP address to connect to.
So we're already relying on DNS, which can be poisoned.
The "fix" for DNS poisoning is not DNS-over-TLS/HTTPS because that basic problem isn't solved.
If we secure DNS against tampering/poisoning, then the same infrastructure can deliver public keys and other information safely as well.