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GlossaryUpdated

What is an email bounce rate?

A bounce rate is the share of sent email that could not be delivered to the recipient server. Hard bounces are permanent failures; soft bounces are temporary. A rising rate is a signal to clean the list and check authentication, not a number to hide by sending more email.

Key facts

  • Mailchimp explains that a bounce is an email that could not be delivered to the recipient. Source
  • RFC 5321 describes SMTP reply codes, including the permanent and transient failure classes that bounces are built on. Source
  • Google asks bulk senders to keep spam rates low and to authenticate their mail, both of which affect whether messages reach the inbox. Source

What a bounce rate means

This definition is part of the Mailprompt glossary. A bounce is an email that could not be delivered to the recipient. The bounce rate is the number of bounces divided by the number of sends, usually expressed as a percentage.

The number is a symptom, not a diagnosis. A high bounce rate can come from an old list, a typo in a domain, a misconfigured sending setup or a recipient server that is temporarily refusing mail. The first job is to find the cause, not to hide the number.

Hard and soft bounces

A hard bounce is permanent. The mailbox does not exist, the domain cannot receive mail or the address is syntactically invalid. Sending again will not help, and continuing to send to the address damages the sender's reputation.

A soft bounce is temporary. The mailbox is full, the server is busy or the message was too large. A retry policy can try again later, but the retries have to be bounded. An address that soft-bounces for weeks is effectively a hard bounce.

The reply codes behind these categories come from SMTP. RFC 5321 describes the permanent and transient failure classes that bounce handling is built on. A platform that treats every bounce the same will either retry permanent failures or discard temporary ones.

Why the rate rises

The most common cause is list age. An address that worked two years ago may belong to a closed account. The second most common cause is a data source that was never verified, such as a purchased list or a giveaway with typo-ridden entries.

A third cause is a sending problem. If the sending domain is not authenticated, a recipient server may reject the message before it reaches the mailbox. That rejection can look like a bounce even though the address is valid.

A sudden spike is usually a data or configuration problem. A slow rise is usually list hygiene. The response is different: investigate the spike, and clean the list for the slow rise.

What to do

Suppress hard bounces immediately and stop retrying them. Check the bounce reason and the domain before you remove a whole segment. If a domain is rejecting all mail, the problem may be authentication rather than the addresses.

For soft bounces, use a bounded retry policy and suppress after a defined number of failures. The policy should be visible in the platform, not hidden in a worker. A retry that runs forever is not persistence; it is noise.

Review the bounce rate alongside the complaint rate and the delivery rate. A low bounce rate with a high complaint rate is not a healthy list. A high bounce rate with a low complaint rate is still a deliverability problem.

Frequently asked questions

What is the difference between a hard and a soft bounce?

A hard bounce is a permanent failure, such as a mailbox that does not exist or a domain that cannot receive mail. A soft bounce is temporary, such as a full mailbox or a server that is temporarily unavailable. Hard bounces should be suppressed; soft bounces may be retried within a bounded policy.

What is a good bounce rate?

There is no universal threshold that fits every list. A small, recently confirmed list should bounce far less than a purchased or aging list. What matters is the trend and the cause. A sudden rise points at a data source, a typo in a domain or a sending problem; a slow rise points at list age.

What should I do when the bounce rate rises?

Pause the affected campaign or flow, check the bounce reasons and the domain authentication, and remove addresses that hard-bounce. Do not keep sending to the same list to see whether the number improves. Repeated sends to invalid addresses damage the sending reputation without reaching anyone.

Sources