It's very challenging to maintain good mailer reputation when you let "the public" use your IP addresses (easier to deal with a dozen enterprise mailers than a million mom-and-pop shops). Some vendors use psychological nudges ("reminder: our contract forbids web-scraped and purchased lists"). Some use technical means to block problematic email addresses. Some buy/scrape lists on the down-low and use them to check if customers are uploading scraped lists...
It's very challenging to maintain good mailer reputation when you let "the public" use your IP addresses (easier to deal with a dozen enterprise mailers than a million mom-and-pop shops). Some vendors use psychological nudges ("reminder: our contract forbids web-scraped and purchased lists"). Some use technical means to block problematic email addresses. Some buy/scrape lists on the down-low and use them to check if customers are uploading scraped lists...