Back to Glossary
Deliverability

Hard Bounce

A permanent email delivery failure caused by an invalid or non-existent email address.

Definition

A hard bounce is a permanent email delivery failure that occurs when an email address is invalid, the domain does not exist, or the receiving server has permanently rejected the message. Unlike soft bounces, hard bounces will never succeed on retry and indicate the address should be removed from your list immediately. Hard bounces return SMTP error codes in the 5xx range.

Why It Matters

Hard bounces are the most damaging type of bounce for sender reputation. They indicate poor list quality and can suggest you are sending to purchased or outdated lists. Email providers closely monitor hard bounce rates as a spam signal. Even 1% hard bounces can trigger reputation warnings.

How It Works

When you send to an invalid address, the receiving server responds with a permanent failure code (5xx series). Common codes include 550 (mailbox not found), 551 (user not local), and 553 (mailbox name invalid). Your ESP should mark these as hard bounces, suppress the address, and prevent any future sends to that address.

Example

Common hard bounce scenarios:

  1. 1[email protected] - typo with double 'h', user meant [email protected]. Returns "550 5.1.1 User unknown"
  2. 2[email protected] - the startup shut down and the domain expired. Returns "550 Domain not found"
  3. 3[email protected] - Mike left the company 6 months ago and his mailbox was deleted. Returns "550 Mailbox does not exist"

All three are permanent failures that will never succeed.

Best Practices

  • 1Remove hard bounces immediately, never attempt to send again
  • 2Use double opt-in to prevent typos and fake addresses at signup
  • 3Verify email addresses with an email validation service before importing
  • 4Monitor your hard bounce rate closely, aim for under 0.5%
  • 5Investigate sudden spikes in hard bounces (may indicate list issues)

Frequently Asked Questions

No. Hard bounces indicate permanent problems. Attempting to send again wastes resources and harms your reputation. Even if a subscriber contacts you claiming their address works, have them re-subscribe through your normal signup process with email verification.

Common causes include: typos in email addresses (gmial.com instead of gmail.com), fake addresses entered to access gated content, old addresses where the account was deleted, non-existent or expired domains, employees who left an organization, or addresses blocked at the server level.