Re: How to prevent, or change priority, of dovecot's FAILed relay-submission to relay's IPv6 address, and submit ONLY/first to IPv4?

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And it's a bad assumption that since the host is dual-stack that all services on it will be.

If a hostname resolves to both an A and AAAA record, it should provides services on both.

Says who/what?

There is no should/must/shall in any internet standard that suggest/implies/requires that.