Most (but not all) email tester and email list cleaning and validation services are generally frowned on by many (if not most) inbox providers and ISPs, and many consider their use to be a sign that you may be a spammer. The rise of list cleaning (also known as "list hygiene") and email tester services, by which we mean email address validation and verification services, has tracked in tandem with the rise of new ways to detect spamming activities, including adding email addresses to mailing lists without consent. It's this last bit, the "adding email addresses to a mailing list without consent" that is the sticking point, and it is that activity which list hygiene services are, for the most part, intended to facilitate. Which is why they are disdained by the email receiving industries (inbox providers, ISPs, and spam filtering services). There's a reason that they are called "list cleaning" services; and if you are building your mailing list with consent then your list won't be dirty and need cleaning. Note that it's important to distinguish these services that offer just these list "hygiene" services, and those who help you to not only clean up your email list, but also to make sure that you are following best practices.