What is a bounce in email?

An email bounce is a message that couldn't be delivered. The receiving server rejected it and sent back an error explaining why.

Bounces are classified into two main types:

Hard bounces are permanent failures. The email address doesn't exist, the domain is invalid, or the recipient server definitively refuses delivery. These addresses should be removed from your list immediately.

Soft bounces are temporary failures. The mailbox is full, the server is temporarily unavailable, the message is too large, or there's a temporary policy block. The address might work later.

Bounce messages contain diagnostic information. SMTP error codes and text explanations tell you what went wrong. A 550 "user unknown" is clear. A 451 "try again later" suggests a temporary issue.

Bounce handling matters for deliverability. Continuing to send to addresses that bounce damages reputation. High bounce rates signal poor list hygiene. Remove hard bounces immediately. Monitor soft bounces and remove addresses that consistently fail.

Your ESP typically processes bounces automatically. Hard bounces are usually suppressed immediately. Soft bounces might be retried a few times before suppression. Understand your platform's policies.