There are several methods to identify non-existent addresses: - Bounce Reports: Use bounce reports provided by your ESP to identify addresses that consistently result in hard bounces. - Validation Tools: Utilize email validation tools to [verify email addresses](href="") before adding them to your list. These tools can detect syntax errors, domain validity, and whether the email address is active. - Double Opt-In: Implementing a double opt-in process can help ensure that the email addresses you collect are valid and belong to engaged users.