Some practical advice here: do not change your canonical domain[1] name unless you really really have to.

If he had just set his fun new domain to redirect to the existing domain, instead of making the new domain the canonical, it likely would have had no negative effect.

I’m not saying this is how things should work. But the practical reality is that your domain name is like a Social Security number: it’s the basis for assigning a type of reputation score, even though it was not intended to do that originally.

[1] The domain at which your web pages finally load, after all redirects have completed.

I don't think it's possible to fix this problem without also helping bad actors. Maybe it's a problem that just isn't worth fixing. Just don't buy preexisting domains unless it's a project big enough to justify the necessary cost of due diligence.

Checking web archive is a basic operation to test if site was hosting anything fishy - not only pirated stuff or porn - often websites has been hacked and changed into link farms or simply were bought on aftermarket simply to use it's SEO value to pass the strength to other domains.

Anyways good point regarding email filters.

I think the mistake here is the redirect old to new. That is always risky so only do it if deseprate. In this case I would have done the redirect from new to old. Then just use the new as a vanity url.