What Is an Email Bounce and How Do You Reduce It?

An email bounce is a message that the receiving server refused or could not deliver. The sender gets a notice, called a bounce message or a non-delivery report, that explains why. Bounces are normal in small numbers and harmful in large ones.

The two types

• Hard bounce: a permanent failure. The mailbox does not exist, the domain is wrong or the server refuses the sender. Stop sending to the address. See the error code library for codes such as 550 5.1.1.
• Soft bounce: a temporary failure. The mailbox is full, the server is busy or the message was delayed. Retry, and stop only if it keeps happening.

Typical causes

1. Typos in addresses entered by hand.
2. Old lists where people left or mailboxes were closed.
3. Purchased or scraped lists.
4. Missing SPF, DKIM or DMARC, which makes servers refuse your mail.
5. A sending IP or domain on a blocklist.
6. Sending too fast from a new domain.

What a healthy rate looks like

Aim for a hard bounce rate that stays well below one percent on a clean list, and keep a close eye on anything above two percent. Providers treat a rising bounce rate as a sign of careless sending.

How to reduce bounces

• Verify before you send. Remove invalid addresses with bulk email verification.
• Check at sign-up. Catch typos while the person is still on the form.
• Authenticate your domain. Use the SPF, DKIM and DMARC checker to confirm.
• Remove hard bounces immediately from every list and automation.
• Warm up new domains gradually.
• Re-check old lists before any big send.

Read the message

The bounce message carries a code and a sentence. Look the code up in the email error codes to learn whether the address is bad or your setup is. A single address can be tested with the free email checker, and plans are on the pricing page.

Reading a bounce message

A bounce message has three parts worth reading: the status code, the address that failed and the explanation from the remote server. The code tells you the type:

• Codes starting with 5 are permanent. Stop sending.
• Codes starting with 4 are temporary. Retry.
• Codes starting with 2 mean success.

The second group of numbers adds detail, for example 5.1.1 for a bad mailbox, 5.2.2 for a full mailbox and 5.7.1 for a policy refusal.

A simple decision table

• 5.1.1, 5.1.2, 5.2.1: remove the address.
• 5.7.1, 5.7.26: fix your setup, not the list. Check SPF, DKIM and DMARC with the checker.
• 4.2.2, 4.4.1, 4.7.0, 4.7.1: retry later; stop only after repeated failures.

What to do after each campaign

1. Export the bounce report from your sending platform.
2. Remove permanent bounces from every list.
3. Move repeated soft bounces to a quiet segment.
4. Look at bounces by domain: many failures at one domain can point to a block or a setup issue.
5. Compare the rate with the previous campaign.

Prevention beats cure

Most hard bounces come from addresses that were never checked. Verify first with bulk email verification, keep typos out with a check at sign-up and re-verify before big sends. The full list of codes with fixes is in the email error codes library.