The SMTP way of checking if an email exists is on the wrong side of CANSPAM, beginning the sending process to check an address is specifically addressed. And this assumes you didn't systematically "guess" the address.
EDIT: Looks like they are indeed doing the SMTP method: https://github.com/reacherhq/check-if-email-exists/blob/a052...
I also found a similar, much bigger service here that appears to have been around for a while: https://emailverification.whoisxmlapi.com/api
There are products that definitely make it past the seed round and sometimes even as public companies before enforcement notice that their entire product runs afoul the law.
> The term "initiate", when used with respect to a commercial electronic mail message, means to originate or transmit such message or to procure the origination or transmission of such message, but shall not include actionsthat constitute routine conveyance of such message.
I also don't think (but with less certainty) that what isn't fully sent counts as a message, as the primary purpose is verifying if the address exists. Message is defined as
> any electronic mail message the primary purpose of which is the commercial advertisement or pro-motion of a commercial product or service