I have no clue about this topic and this comment isn't representative of my employer whatsoever, but...
Since you've never experienced this with other domains, does the word cheap have to do with it? When I think domains, "cheap" instantly reminds me of those early quasi-legal online pharmacies and if I see it in an email almost certainly it's spam of one sort of another. I could see being problematic in spam filters.
It's a fairly common English expression that's used especially when giving a quote for development of some product or service to a client (or manager): "Good, cheap, and fast: pick two".
Cheap is just the everyday word for low cost. It can also mean low quality, but that connotation depends more on context.
Besides, there are many other successful companies with "cheap" in their domain names.
I'm well aware, but a bayesian spam filter probably is not. If you look up lists of words most likely to trigger spam filters, "cheap" will be in every one of them. I can't say that is what's happening here but it seems plausible at least.
I would also argue that spam messages aren't really trying to convince you something is low quality, it's precisely the low cost aspect that they're to convince you of even if the product is actual low quality.
I'm sure there's other companies with cheap in their domains/headers, but none stand out and I would have no idea if they depend on SEO anyway.
Since you've never experienced this with other domains, does the word cheap have to do with it? When I think domains, "cheap" instantly reminds me of those early quasi-legal online pharmacies and if I see it in an email almost certainly it's spam of one sort of another. I could see being problematic in spam filters.