Why do emails bounce? Hard and soft bounces explained

6 min read · Updated 23 October 2026

A bounce is an email that could not be delivered and came back to you with an error. A few bounces are normal. Too many tell email providers you send to bad lists, and your messages start going to spam, even for people who want them.

Hard bounces

A hard bounce is a permanent failure. Sending again will not help. Common causes:

• The mailbox does not exist. The person left the company, or the address was mistyped or guessed wrong.
• The domain does not exist or has no mail server, for example a typo like gmial.com or a company that closed.
• The receiving server blocked you permanently, for example because of a bad sender reputation.

Remove hard-bounced addresses from your list straight away and never send to them again.

Soft bounces

A soft bounce is a temporary failure. The address may work later. Common causes:

• The mailbox is full.
• The receiving server is busy or down.
• The message is too large.
• The server is slowing you down (greylisting or rate limits), often when you send many messages at once.

Most sending tools retry soft bounces for a while. If the same address soft-bounces again and again, treat it as a hard bounce.

Common bounce messages and what they mean

• 550 5.1.1 User unknown / No such user: the mailbox does not exist. Hard bounce.
• 550 5.1.2 Domain not found: the domain has no mail server. Hard bounce.
• 552 5.2.2 Mailbox full: the inbox is full. Soft bounce.
• 421 / 450 Try again later: the server is busy or greylisting you. Soft bounce.
• 550 5.7.1 Message rejected / blocked: your message or server was refused, often because of reputation or missing authentication. Check your SPF, DKIM and DMARC.

What is a good bounce rate?

Aim for under 2 percent per campaign. Large mailbox providers watch bounces and spam complaints closely, and a rate above about 5 percent is a sign your list needs cleaning.

Seven ways to reduce bounces

1. Verify every new address before it goes into your list, for example with a sign-up form check.
2. Clean old lists before a campaign. Addresses go bad as people change jobs. See how to clean an email list.
3. Catch typos like gmial.com or yaho.com at the point of entry.
4. Never buy lists. They are full of dead addresses and spam traps.
5. Send catch-all addresses in a separate, small batch. See what catch-all means.
6. Set up SPF, DKIM and DMARC so receiving servers trust your mail. Check them with the free domain tools.
7. Remove bounces automatically. Most email tools can suppress hard bounces for you. Turn it on.

Domailfix verifies addresses without sending any email, so checking a list never adds bounces of its own.

Clean your list free before your next send.

Questions and answers

Do bounces hurt my email reputation?

Yes. A high bounce rate tells mailbox providers you are sending to old or bought lists, which can push all your emails to spam.

Should I remove soft-bounced addresses?

Not after one soft bounce. If the same address soft-bounces in several campaigns, remove it.