Usually when this happens, it means the email address is either invalid or the recipient blocked all emails from that sender. When a hard bounce occurs, some email service providers automatically remove that recipient from their mailing list so it doesn’t happen again in the next campaign.

Too many hard bounces can be a red flag for spam filters. In order to make sure your messages don’t get marked as spam, you need to constantly maintain your email list.

You can do that by:

  • Avoiding bought email lists
  • Making sure your emails are relevant
  • Not using free sender domains
  • Removing inactive subscribers
  • Using a signup form with a good captcha system

By following these guidelines, you can prevent getting blacklisted and maintain a strong email list, which is necessary for successful email marketing campaigns.