Deliverability
Hard Bounce
A permanent email delivery failure caused by an invalid address, non-existent domain, or server rejection.
A hard bounce is an email that has been permanently rejected by the recipient's mail server. The most common causes are:
- Non-existent mailbox: The email address doesn't exist (user has left the company, typo in address)
- Invalid domain: The domain has no MX records or has expired
- Server rejection: The recipient's server has explicitly blocked your sending domain
- Policy rejection: The email violates the recipient server's policies
Hard bounces should be immediately removed from your email list. Continuing to send to hard-bounced addresses damages your sender reputation and can lead to blacklisting.
SendSure's verification engine catches hard bounces before you send by performing SMTP handshakes that confirm mailbox existence without actually sending an email.
Related Blog Posts
Want to learn more?
Read our in-depth blog posts on email verification and deliverability.