Thanks for the feedback. 'Had this same problem [not with Hostinger]. It stopped when I started registering domains with privacy. Looks like spammers pull the email address from WHOIS.
That's what Hostinger said when I contacted their support, but I literally did not have this problem with 100+ domains registered with other registrars (GoDaddy, Gandi, TUCOWS, Namecheap, Amazon, etc.) - it was just Hostinger. My suspicion is that they sell your information when you register a domain; I have no other explanation for the phenomenon.