"These TXT records additionally contain a policy "id" field, allowing Sending MTAs to check that a cached policy is still current without performing an HTTPS request."
So, yes, it is "skip that HTTP request".
Despite HTTP(S) has caching rules defined, they are just different from "caching" logic incorporated in MTA-STS. Key differences are:
1. Sending party ought to update MTA-STS policy for recipient domain far before it expires.
2. Sending party can check if policy is still actual without direct contact with policy server by mean of retrieving id in TXT record. That significantly reduces amount of requests policy server has to serve in order to keep all senders up to date with its latest STS policy. Normally, HTTPS requests occur only once from each sender per policy version.
3. Sending party must reuse cached policy if STS policy of recipient suddenly disappeared without trace or failed to fetch for other reasons.
In a nut shell, "cache" and "max-age" in MTA-STS has different interpretation than in HTTPS and require some additional (obscure) logic for processing.
"These TXT records additionally contain a policy "id" field, allowing Sending MTAs to check that a cached policy is still current without performing an HTTPS request."
So, yes, it is "skip that HTTP request".
Despite HTTP(S) has caching rules defined, they are just different from "caching" logic incorporated in MTA-STS. Key differences are: 1. Sending party ought to update MTA-STS policy for recipient domain far before it expires. 2. Sending party can check if policy is still actual without direct contact with policy server by mean of retrieving id in TXT record. That significantly reduces amount of requests policy server has to serve in order to keep all senders up to date with its latest STS policy. Normally, HTTPS requests occur only once from each sender per policy version. 3. Sending party must reuse cached policy if STS policy of recipient suddenly disappeared without trace or failed to fetch for other reasons.
In a nut shell, "cache" and "max-age" in MTA-STS has different interpretation than in HTTPS and require some additional (obscure) logic for processing.