I receive a few hundred thousand emails per month across my various domains, with some domains attracting similar numbers. The main cause seems to be that the domain was previously used for one or more email address that has ended up being published somewhere quite popular.
For example, I own a domain that previously belonged to a ~20 year old pharmacy services business that was eventually acquired and renamed. The domain has appeared in many places online as an email address, like press releases. I've received ~100k emails to that domain.
Another possibility is that, for whatever reason, someone has been using the domain to sign up to various services when typing
randomly. One of my domains is the sort of thing someone might type when trying to come up with a fake email address, so that gets
a lot of spam to
obviously fake email addresses.
Looking at your domain, I can see it still appears in various places as an email address, including the contact page for
HRC Hotels and on
NextDoor. I'd guess most of the emails are to addresses that were once in use, and because those addresses have appeared frequently online, they're in lots of spam lists.
Something important to note is that catch-all email systems may accept anything[1], including email that would be rejected by a "real" email system. So, you're not necessarily seeing the same email you would see if you had set the domain up with Gmail or Outlook, you're seeing junk that wouldn't even make it as far as Gmail's junk folder because it is obviously not legitimate email.
[1] Depends on the system so there may be
some anti-spam in place in whichever system you use. For my system, I accept everything.