What is a hard bounce email?

A hard bounce email is an email that can’t be delivered to a recipient’s inbox because of a permanent issue. A high number of bounced emails can jeopardize your sender reputation, as spam filters often see this as a warning sign that there are potential issues with the sender. 

Hard bounce emails can be more detrimental to your sender reputation than soft bounce emails, so cleaning your email lists is important for successful email marketing.

Why do hard bounces happen?

There are several reasons why an email can bounce, but these are the most common for hard bounces:

1. The recipient’s email address doesn’t exist

Hard bounces often indicate that the entered recipient’s email address doesn’t exist or is invalid. The error can happen when:

  • There’s a typo in the email address
  • The domain doesn’t exist
  • The recipient changed their email address and deleted the old one 

When your email service provider (ESP) tries to deliver a message to a non-existent email address and it hard bounces, you’ll get a 550 5.1.1 error. You can avoid this issue by using a double opt-in for sign-ups, where a subscriber needs to confirm their subscription to your list by clicking on an automated confirmation message.

2. The email was blocked by the server

Most modern email servers reject a substantial volume of emails that they suspect are spam. And sometimes, your emails may get caught in the crossfire. If your emails are permanently blocked by a server, it’s most likely due to improper email authentication. Before you start sending emails, it’s important to make sure that your SPF, DKIM, and DMARC protocols are set up correctly.

3. The mailbox is full

When a recipient’s mailbox is full, their email server won’t be able to accept any more messages, causing your email to hard bounce. This situation often indicates that the account is abandoned or not actively monitored.

If an email consistently bounces due to a full mailbox, it’s a good indicator that you can safely remove the address from your list to maintain your sender reputation and improve deliverability rates.

How to deal with hard bounce emails

  1. Clean your email lists regularly: Create exclusion lists for soft bounces, and use a marketing automation platform that automatically deletes hard bounces from your lists, so it’s easy to maintain a healthy sender reputation.
  2. Use a double opt-in: A double opt-in mechanism confirms that all subscribers have a valid email address before you start sending to them.
  3. Avoid free sender domains: Don’t use free services like Gmail, Outlook, or Yahoo to send bulk emails and manage your lists (something Klaviyo doesn’t even allow). Send emails from a branded sender domain that has been properly authenticated.

Marketing automation platforms like Klaviyo can help keep your email lists clean.

With Klaviyo, you can monitor the deliverability of your emails by reporting on:

Sign up for Klaviyo today to make sure your emails always land in the right inboxes.

Additional resources