What Are Email Bounces?
An email bounce occurs when a message you send cannot be delivered to the intended recipient. The receiving mail server rejects the message and sends back a notification (a "bounce message" or "Non-Delivery Report") explaining why delivery failed.
Bounces are a normal part of email operations. Even with perfect list hygiene, some small percentage of messages will bounce due to temporary server issues or recipients who change email addresses. The goal is not to eliminate bounces entirely, but to keep your bounce rate within healthy limits.
Your bounce rate is calculated as:
Bounce Rate = (Number of Bounced Emails / Number of Emails Sent) x 100Hard Bounces vs Soft Bounces
Bounces are categorized into two types based on whether the failure is permanent or temporary.
Hard Bounces
A hard bounce is a permanent delivery failure. The message can never be delivered to that address. Common causes include:
- The email address does not exist (typo, fake address, or deleted account)
- The domain does not exist or has no mail servers (no MX records)
- The receiving server has permanently blocked your sender
Hard bounces are the most damaging type of bounce for your sender reputation. They signal to mailbox providers that you are sending to addresses you should not be sending to, which suggests poor list quality or list acquisition practices.
Soft Bounces
A soft bounce is a temporary delivery failure. The address is valid, but something prevented delivery at that moment. Common causes include:
- Recipient's mailbox is full
- Receiving server is temporarily unavailable or overloaded
- Message is too large for the recipient's server
- Temporary DNS resolution failure
- Receiving server rate-limiting your sending IP
- Content-based rejection (spam filtering)
Soft bounces are less immediately damaging than hard bounces, but consistent soft bounces to the same address over multiple sends should eventually be treated as a hard bounce.
Comparison Table
| Factor | Hard Bounce | Soft Bounce |
|---|---|---|
| Duration | Permanent | Temporary |
| Cause | Invalid address, non-existent domain | Full mailbox, server down, rate limiting |
| Retry worthwhile? | No - remove immediately | Yes - retry a few times, then remove |
| Reputation impact | Severe | Moderate (if persistent) |
| Acceptable rate | Below 2% | Below 5% (combined with hard) |
| Action required | Remove from list after 1st bounce | Remove after 3-5 consecutive soft bounces |
Common Bounce Codes and What They Mean
Bounce messages include SMTP status codes that explain why delivery failed. These codes follow a standard format defined in RFC 3463. Here are the most common codes you will encounter:
5xx Codes (Hard Bounces)
| Code | Enhanced Code | Meaning |
|---|---|---|
| 550 | 5.1.1 | User unknown / mailbox does not exist |
| 550 | 5.1.2 | Domain not found / bad destination |
| 550 | 5.1.0 | Address rejected (general) |
| 550 | 5.7.1 | Message rejected by policy (sender blocked) |
| 551 | 5.1.6 | Recipient has moved / address no longer valid |
| 552 | 5.2.2 | Mailbox full (can be hard if persistent) |
| 554 | 5.7.1 | Transaction failed / message refused |
4xx Codes (Soft Bounces)
| Code | Enhanced Code | Meaning |
|---|---|---|
| 421 | 4.7.0 | Connection rate limited / try again later |
| 450 | 4.2.1 | Mailbox temporarily unavailable |
| 451 | 4.3.0 | Temporary system error on receiving server |
| 452 | 4.2.2 | Mailbox full (temporary) |
| 450 | 4.7.1 | Greylisting - try again in a few minutes |
How Bounces Affect Sender Reputation
Bounces directly impact your sender reputation through several mechanisms:
Direct Reputation Scoring
Mailbox providers track your bounce rate over time. High bounce rates indicate poor list quality, which correlates strongly with spamming behavior. Google, Microsoft, and other major providers factor bounce rates into their sender reputation algorithms.
Blacklist Risk
Consistently high bounce rates can trigger blacklist listings. Blacklist operators monitor for senders who show signs of sending to unverified lists, and a high hard bounce rate is one of the clearest indicators. Check your blacklist status with our Blacklist Checker.
ESP Account Standing
Email Service Providers monitor the bounce rates of their customers. Excessive bounces can result in account warnings, throttling, or even account suspension. Most ESPs have bounce rate thresholds (typically 2-5%) that trigger automatic review.
Cascading Effects
High bounce rates lead to lower sender reputation, which leads to more messages being filtered to spam, which leads to lower engagement rates, which further damages your reputation. It becomes a negative cycle that gets harder to escape the longer it continues.
Acceptable Bounce Rate Benchmarks
Industry benchmarks vary by email type and sector, but here are general guidelines:
| Bounce Rate | Assessment | Action Required |
|---|---|---|
| Below 1% | Excellent | Maintain current practices |
| 1% - 2% | Acceptable | Monitor and maintain list hygiene |
| 2% - 5% | Concerning | Review list quality and acquisition methods |
| 5% - 10% | Poor | Immediate action required, clean your list |
| Above 10% | Critical | Stop sending and perform a full list audit |
These benchmarks apply primarily to hard bounces. Total bounce rate (hard + soft) should generally stay below 5%. Transactional email (order confirmations, password resets) typically has lower bounce rates than marketing email because the addresses were recently active.
Reducing Your Bounce Rate
Here are proven strategies to bring your bounce rate down and keep it low:
Implement Double Opt-In
Double opt-in requires new subscribers to confirm their email address by clicking a link in a confirmation email. This prevents typos, fake addresses, and bot signups from entering your list. It is the single most effective measure for reducing bounce rates.
Validate at Point of Entry
Add real-time email validation to your signup forms. Check for:
- Proper email format (contains @ symbol, valid characters)
- Valid domain (MX records exist for the domain)
- Common typos (gmail.con, hotmial.com, etc.)
- Disposable email addresses (if appropriate for your use case)
Clean Your List Regularly
Run your full email list through a verification service periodically (quarterly at minimum). These services check each address against multiple data sources and flag addresses that are likely to bounce. Remove flagged addresses before your next send.
Process Bounces Immediately
Configure your ESP or mail server to automatically remove hard-bounced addresses after the first occurrence. For soft bounces, set up rules to remove addresses that soft-bounce consistently across 3-5 consecutive sends.
Remove Inactive Subscribers
Subscribers who have not engaged with your emails in 6-12 months are more likely to have abandoned their email account. Abandoned accounts eventually become invalid and generate hard bounces, or worse, get recycled into spam traps. Run a re-engagement campaign and remove non-responders.
Monitor Acquisition Sources
If you collect email addresses from multiple sources (website forms, events, partner lists, social media), track bounce rates by source. If one source consistently produces higher bounce rates, investigate and fix the issue or stop using that source.
Bounce Handling Best Practices
Automate Bounce Processing
Your email infrastructure should automatically process bounce notifications. At minimum:
- Hard bounces trigger immediate suppression (address is blocked from future sends)
- Soft bounces trigger retry logic (retry 2-3 times over 24-72 hours)
- Persistent soft bounces (3-5 consecutive sends) trigger suppression
- Suppressed addresses are stored in a suppression list to prevent accidental re-addition
Maintain a Suppression List
Keep a permanent record of all bounced addresses. Even if someone tries to re-subscribe with a previously bounced address, check it against your suppression list first. Addresses should only be un-suppressed if the subscriber confirms via double opt-in that the address is now valid.
Monitor Bounce Patterns
Watch for unusual patterns in your bounce data:
- A sudden spike in bounces may indicate a problem with a specific list segment or data import
- Bounces concentrated at one provider (e.g., all Gmail bounces) may indicate you have been blocked by that provider
- Increasing soft bounce rates may indicate your IP or domain reputation is declining
Use Feedback Loops
Sign up for feedback loops with major mailbox providers. While feedback loops primarily report spam complaints (not bounces), the complaint data helps you identify recipients who do not want your email, which prevents future bounces when those addresses eventually become invalid.