The "mx" field is required, a policy without it is invalid, so add a
check for it.
See
https://mailarchive.ietf.org/arch/msg/uta/Omqo1Bw6rJbrTMl2Zo69IJr35Qo
for more background, in particular the following paragraph:
> The "mx" field is required, so if it is missing, the policy is invalid
> and should not be honored. (It doesn't make sense to honor the policy
> anyway, I would say, since a policy without allowed MXs is essentially a
> way of saying, "There should be TLS and the server identity should match
> the MX, whatever the MX is." I guess this prevents SSL stripping, but
> doesn't prevent DNS injection, so it's of relatively little value.)
This EXPERIMENTAL patch has a basic implementation of MTA-STS (Strict
Transport Security), based on the current draft at
https://tools.ietf.org/html/draft-ietf-uta-mta-sts-02.
It integrates the policy fetching and checking into the smtp-check tool
for convenience, but not yet in chasquid itself.
This is a proof of concept. Many features and tests are missing; in
particular, there is no caching at all yet.
