Sending email from a decoy, pass-through email domain which forwards to a primary domain is never a good idea. (Some people call this a 'dummy domain', but that's actually something different.) What they do is set up a decoy domain and send their cold email from it, with links in the cold email which point to their primary domain. They do it this way in an effort to protect the reputation of their primary domain, aren't they so clever? Here's the thing; actually two things: 1. It doesn't work, it will still drag down their primary domain's reputation, and 2. it doesn't work because they are spamming. Calling it "cold email" when what you've done is scraped or purchased an email address and put it on a mailing list without consent is spam, no matter how much you try to polish it up and call it something else.
