Yeah, it'd be great if there were test target SMTP servers with all useful configurations to test relay SMTP server behaviour in regards to DANE. I am now defi

Yeah, it'd be great if there were test target SMTP servers with all useful configurations to test relay SMTP server behaviour in regards to DANE. I am now definitely curious, though I can't promise I'll find the time to set up the servers either. DANE on top of DNSSEC is merely a policy (retry rules and no fallback to unencrypted comms), so it is relatively simple to implement — though you are right that this doesn't mean it is! I think adding gmail as the largest email provider to your list of mail services to check is also useful.