# Managing email bounces

> The different bounce types in MailMachine - soft, hard, blocked, full mailbox and more - and how to handle each.

Source: https://www.kualo.com/knowledgebase/senders-deliverability/managing-email-bounces
Updated: 2026-06-10

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A bounce is a failed delivery attempt. A **soft bounce** is a temporary failure that delays delivery but keeps the contact on your list; a **hard bounce** means the address is unreachable and is removed. Knowing the different bounce types helps you read your reports and keep your list healthy. For the metrics and benchmarks, see **[Bounces, complaints and unsubscribes](/knowledgebase/mailmachine-reporting/bounces-complaints-and-unsubscribes)**.

## The bounce types you'll see

**Soft bounce** - the message reached the recipient's server but came back undelivered, often a temporarily full inbox. Repeated soft bounces to the same address eventually become a hard bounce.

**Transient bounce** - the sending server is still retrying; it keeps trying for up to two days. Usually nothing to worry about. You may see "message still undelivered after 4 hours".

**Address change** - the recipient has moved to a new address and the server is letting you know.

**Auto-reply** - an out-of-office or similar automated response. Not a real bounce; safe to ignore, though useful to know if your message was time-sensitive.

**Challenge/Response** - filtering software that only accepts mail from known senders has asked the sender to complete a challenge before the message is delivered.

**DNS failure** - a temporary DNS problem stopped delivery.

**Hard bounce** - the address is invalid: the domain doesn't exist, or the mailbox is unknown. Removed from your list.

**Mail blocked** - the recipient's server is refusing your mail (you might see "550 Message REFUSED by peer" or "552 Blocked by filters").

**Full mailbox** - the recipient's inbox is full, so the server can't deliver.

## Troubleshooting

**Hard bounce rate suddenly jumped** - usually a freshly imported list that wasn't validated, or typos in manually added addresses. Validate your list before the next send.

**The same addresses keep soft-bouncing** - watch whether they turn into hard bounces, check if a whole domain is bouncing (which can mean blocking), and consider removing addresses that soft-bounce across several campaigns.

**Lots of "full mailbox" bounces** - these accounts may be abandoned. Run a re-engagement campaign, and drop addresses that stay full for a month or more.

**Challenge/Response is blocking delivery** - only worth completing the challenge for genuinely high-value contacts; otherwise these are high-effort to maintain.

If bounces spike for no obvious reason, [open a support ticket](/knowledgebase/getting-started/how-to-create-a-support-ticket-in-mykualo).

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_Source: Kualo Knowledgebase — https://www.kualo.com/knowledgebase/senders-deliverability/managing-email-bounces · © Kualo Ltd._