Bounced Emails

What are Bounced Emails?

Bounced emails are messages that the recipient's mail server refused to accept and returned to the sender. The rejection comes with an SMTP status code and a textual reason, which the sending platform parses to decide whether to retry, suppress, or investigate.

Why it matters

Every bounce is a signal. Ignored bounces stack up into reputation damage; categorized bounces drive automatic list cleanup and give you visibility into what is wrong. Developers, marketers, and deliverability engineers all rely on bounce data to keep lists clean and inbox placement strong.

How it works

When a receiving server rejects a message, it responds with a 4xx code for temporary failures or 5xx for permanent ones. The sending server generates a Delivery Status Notification (DSN) and returns it to the envelope sender. Platforms like Sendkit parse the DSN, classify the bounce as hard, soft, block, or out-of-office, and update the suppression list accordingly without human intervention.

Examples

• A hard bounce with "550 5.1.1 User unknown" added immediately to suppressions
• A soft bounce with "452 4.2.2 Mailbox full" retried automatically for 48 hours
• A block bounce with "554 5.7.1 Blocked by policy" flagging a reputation issue

Best practices

• Parse the SMTP reply code, not just the text, for reliable classification
• Retry soft bounces with exponential backoff, then suppress if they persist
• Alert on sudden bounce rate increases; they often signal infrastructure problems
• Let Sendkit handle bounce categorization automatically instead of parsing DSNs by hand

FAQs

What is the difference between a bounce and a block?

A bounce covers all rejection reasons. A block is a specific bounce subtype where the receiver is refusing because of policy, content, or reputation rather than an invalid mailbox.

How many bounces are too many?

Above 2 percent is a warning for marketing, above 5 percent is urgent. Transactional mail should stay well below 1 percent.

Should I email a bounced address again later?

If the bounce was soft and the cause resolved (server downtime, full mailbox), retrying is fine. Never retry hard bounces.