# Bounce Rate

## The percentage of emails that fail to deliver. Hard bounces are permanent, soft bounces are temporary.

## What is Bounce Rate?

Bounce rate measures the percentage of emails that fail to deliver. It’s a key metric for list health and deliverability. Bounces are divided into two categories: **hard bounces** (permanent failures like invalid addresses) and **soft bounces** (temporary failures like a full mailbox).

### Why it matters

High bounce rates signal poor List Hygiene and can severely damage your sender reputation. ISPs interpret high bounce rates as a sign of spammy behavior, which can lead to blocking or filtering. Monitoring bounce rate helps you catch problems before they spiral.

### How it works

When an email fails, the receiving server sends back an [SMTP](/content/glossary/smtp-simple-mail-transfer-protocol "SMTP (Simple Mail Transfer Protocol/index.html)") error code. A "user not found" error is a hard bounce. A "mailbox full" error is a soft bounce. [ESPs](/content/glossary/email-service-provider-esp "Email Service Provider (ESP/index.html)") like AutoSend automatically add hard bounces to your [Suppression List](/content/glossary/suppression-list "Suppression List"/index.html) to prevent further harm.

### Examples

- Sending to a purchased list full of invalid emails (hard bounces).
- A holiday campaign landing in full inboxes (soft bounces).
- Old corporate addresses bouncing after employees leave.

### Best practices

- Validate emails at collection to prevent typos.
- Regularly clean your lists to remove inactive or invalid addresses.
- Monitor bounce reports to detect systemic issues.

### Related terms

[Bounced Emails](/content/glossary/bounced-emails "Bounced Emails"/index.html), [Invalid Emails](/content/glossary/invalid-emails "Invalid Emails"/index.html), [Suppression List](/content/glossary/suppression-list "Suppression List"/index.html)

### FAQs

**What’s an acceptable bounce rate?**  
Under 2% is generally considered healthy. Anything higher signals problems with acquisition sources, data quality, or technical setup. Persistent high bounce rates can lead to blacklisting.

**What’s the difference between hard and soft bounces?**  
Hard bounces are permanent (invalid domain, user doesn’t exist). Soft bounces are temporary (mailbox full, server unavailable). ESPs typically retry soft bounces for a period before treating them as failures.

**How do I reduce bounce rate?**  
Focus on prevention: validate addresses at signup, use double opt-in, and avoid buying lists. Also, monitor engagement and suppress addresses that haven’t interacted with your emails for months.