Use <service>@<yourdomain> as your email address when signing up, and check the To header when receiving emails.
And/or, long-press or right-click on any link to inspect the linked domain.
What fraction of people do you suppose actually have a <yourdomain> to do this with?
Even some highly technically inclined people (like myself) can be entirely ignorant of the process. It's not as if consumer ISPs provide the service.
If you don't control your own domain fully, almost all email services let you do:
And have it go to [email protected] with the servicetag still in the To: field. At least, I have never encountered a problem with this.
I often go one step futher by appending a short random identifier, `{service}.{id}@{domain}`, to make it harder to guess (in case someone learned of my email address policy).
I created a little GTK program to help: https://github.com/LightAndLight/gen-alias