There are multiple methods to identify invalid emails manually:
Bounce Reports: Regularly check bounce reports from your email service provider. Hard bounces indicate permanent issues with the email address, such as it being invalid or non-existent. Engagement Metrics: Identify recipients who haven't opened or clicked any emails over an extended period. These users may have abandoned their email accounts. Feedback Loops: Monitor feedback loops for any complaints or spam reports. Frequent complaints may indicate that the emails are being sent to incorrect or unwanted addresses.