Anymail finder uses many approaches to find emails—it searches billions of web pages and performs direct server validation.
The original SMTP spec allows for email address validation, and there are tricks like opening an SMTP connection to a mail server and dropping it half way if the address is verified -- but these are the same "tricks" that spammers use, so many mail servers disable or report false positives. There's a reason why most lead services have a high price: they have actually verified an email address.
Next, sending cold emails to business is OK (sometimes annoying but legally ok), but the copy on makesmail.com has a broken link (1) and doesn't clearly describe how to cold email and be legally compliant. From the horses mouth: https://www.ftc.gov/tips-advice/business-center/guidance/can...
Regardless, congratulations on building up to $1,500 MRR, that is a milestone most side projects never reach!