Hard Bounce vs Soft Bounce: What Is the Difference?

When an email cannot be delivered, the receiving mail server sends back a bounce message. Email platforms sort these into two groups: hard bounces and soft bounces. Knowing the difference tells you which addresses to delete and which ones to try again.

What is a hard bounce?

A hard bounce is a permanent delivery failure. Sending again will never work. The most common causes are:

• The mailbox does not exist (the person left the company, or the address was mistyped).
• The domain does not exist or has no mail servers.
• The receiving server has permanently blocked your messages.

Hard bounces should be removed from your list straight away. They hurt your sender reputation more than anything else, because they suggest you are mailing a list you did not collect carefully.

What is a soft bounce?

A soft bounce is a temporary failure. The address is real, but the message could not be delivered right now. Typical reasons:

• The mailbox is full.
• The receiving server is down or busy.
• The message is too large.
• The server is limiting how many messages it accepts from you (throttling or greylisting).

Most email platforms retry soft bounces automatically for a day or two. If the same address soft bounces on several campaigns in a row, treat it like a hard bounce and remove it.

Common bounce codes

Bounce messages include a code. Codes starting with 5 are permanent (hard); codes starting with 4 are temporary (soft).

How to prevent both kinds of bounce

You cannot control a full mailbox, but you can stop almost every hard bounce before it happens. Verifying email addresses checks whether each mailbox exists, so invalid ones are removed before you send. For soft bounces, keep your sending volume steady, authenticate your domain with SPF, DKIM and DMARC, and avoid very large attachments.

For a full plan, read how to reduce your email bounce rate.