To be honest, stuff not working when it breaks the standard is unfortunate, but I wouldn’t blame this on the tool that adheres to said standard.
You’re not inconvenienced by systemd-resolvd, you’re inconvenienced by those mail sites doing stuff that doesn’t work, possibly as a result of them needing to do something that was slightly flawed to begin with: using DNS records to possibly hold more data than they can per the spec, which, if I understand things correctly, is because of the limitations of UDP traffic.
Not that that helps you, of course, it’s annoying and I recognise that.
I, too, live in this fabled real world, and I already mentioned I understand your issue. I just think you’re barking up the wrong tree, but luckily you’re able to work around things, and that’s the most important bit, isn’t it?