SSL externally points to 123.45.67.8 Your internal DNS responds on 10.20.30.40 SSL very works fine as it validates Mail.mycorp.com, not the IP This fails as

SSL externally points to 123.45.67.8 Your internal DNS responds on 10.20.30.40 SSL very works fine as it validates Mail.mycorp.com, not the IP This fails as DoH becomes more common and your dhcp server saying “use dns 10.1.1.1” is ignored, but then connections to your mail server fail too.