A prospect who gets your email and types the domain into a browser expects a website. So does every reputation service that checks. A secondary domain serving nothing is a signal — and the signal is "throwaway".
Sending domains can now serve a redirect landing page: a real page on the domain that forwards visitors to your main site. Set it per domain, or in bulk across all of them.
The pages are protected against automated abuse, so they cannot be scraped or hammered into becoming a reputation problem themselves. The domain detail page shows the DNS needed to enable it, and whether it is currently live.