What happens if I don't manage hard bounces?

Ignoring hard bounces will severely damage your sender reputation, leading to lower inbox placement, increased spam complaints, and potential blacklisting (or blocklisting) . Your emails may also be throttled or completely blocked by ISPs.

How many soft bounces before it becomes a hard bounce?

This depends on the Email Service Provider (ESP) and the recipient's mail server. Many ESPs automatically attempt to resend soft bounced emails a few times (e.g., 3-5 times) over a period of days. If the email continues to soft bounce after these attempts, it may be converted into a hard bounce and removed from your list, as it indicates a persistent problem. Soft bounce suppression logic varies.

Can I resend emails to soft bounced addresses?

Yes, it is generally acceptable to resend emails to soft bounced addresses, as the issue is typically temporary. Most ESPs handle this automatically by retrying delivery over a short period. However, if an address consistently soft bounces over multiple campaigns or retries, it should be considered for removal to maintain list health.

How do hard and soft bounces affect my email deliverability?

Both hard and soft bounces, especially when frequent, negatively impact your email deliverability. Hard bounces immediately signal poor list quality and can lead to direct blocklisting. While soft bounces are temporary, a high rate suggests a less engaged or problematic list, which can gradually erode your sender reputation and lead to increased spam filtering over time. Understanding bounce rate is key.

What is the difference between hard and soft email bounces and how should they be managed? - Technical - Email deliverability - Knowledge base

Email deliverability is a complex landscape, and understanding how emails fail to reach their intended recipients is crucial for maintaining a healthy sending reputation. When an email cannot be delivered, it results in a bounce. Not all bounces are created equal, however. The distinction between hard and soft bounces is fundamental for any sender looking to optimize their email campaigns and avoid deliverability pitfalls.

These bounce types signal different underlying issues and demand varying management strategies. Mismanaging bounces can lead to a damaged sender reputation, higher bounce rates, and even cause your emails to land in spam folders or be blocked entirely. Ignoring bounces is not an option if you want to ensure your messages consistently reach the inbox.

By correctly identifying and addressing both hard and soft bounces, you can maintain clean email lists, improve your inbox placement, and foster better engagement with your audience. Let's delve into what each bounce type signifies and the best practices for handling them to safeguard your email program.

What is a hard bounce?

A hard bounce indicates a permanent reason why an email cannot be delivered to a recipient. This means the email address is no longer valid, does not exist, or the server has definitively rejected the email. When a hard bounce occurs, the mail server sends back a Non-Delivery Report (NDR) with an SMTP error code, typically starting with 5xx. It signals that future attempts to send to that address will also fail.

Common SMTP 5xx (hard bounce) codes

550: Mailbox not found
550: User unknown
550: No such user here
553: Recipient address rejected: User unknown in local recipient table

The main causes of hard bounces include a non-existent email address, an invalid domain name, or the recipient's mail server permanently blocking your sending IP or domain. For example, if you send to nonexistent@example.com, it will result in a hard bounce because the recipient does not exist. Similarly, sending to a domain that is no longer active will also cause a hard bounce.

The impact of hard bounces on your sender reputation is significant and immediate. Each hard bounce signals to mailbox providers like

Google and

Yahoo that your email list may be outdated or poorly managed. A high hard bounce rate can lead to your emails being marked as spam or your sending IP address being placed on a blacklist (or blocklist). For these reasons, hard bounced email addresses should be removed from your mailing list immediately and permanently to protect your sender reputation and deliverability.

What is a soft bounce?

In contrast to hard bounces, a soft bounce indicates a temporary email delivery issue. This means the email address is valid, and the message reached the recipient's mail server, but it was temporarily rejected for a specific reason. The server typically sends back an SMTP error code that begins with 4xx, suggesting that the issue might resolve itself, allowing for future delivery attempts.

Common causes of soft bounces include a full inbox, meaning the recipient has exceeded their storage limit, or the recipient's server being temporarily down or offline. Another reason could be that the email message is too large for the recipient's server to accept, or that the mailbox provider has identified a temporary issue that is not permanent. Unlike hard bounces, soft bounces indicate a problem that might go away, allowing for future delivery attempts.

While soft bounces are less severe than hard bounces, a high volume of them or repeated soft bounces to the same address can still negatively impact your sender reputation. Many Email Service Providers (ESPs) will attempt to resend an email that soft bounces a few times over a period. If the email continues to soft bounce after several attempts, some ESPs may convert it into a hard bounce and remove the address from your list. This is why it's important to monitor them, even if they aren't immediately catastrophic.

Identifying soft bounce codes

Soft bounces are typically indicated by SMTP error codes starting with 4xx. Some common examples include:

421: Service not available, closing transmission channel
450: Requested mail action not taken: mailbox unavailable or busy
451: Requested action aborted: local error in processing
452: Requested action not taken: insufficient system storage

Management strategies

Effective bounce management is critical for maintaining high email deliverability. The way you handle hard versus soft bounces should differ significantly due to their underlying nature. Proper bounce management, including bounce suppression logic, is key to preserving your sender reputation and ensuring your messages reach valid inboxes.

Hard bounce management

Immediate removal: Addresses that hard bounce should be removed from your mailing list permanently. Attempting to resend to these addresses will only harm your sender reputation.
List hygiene: Implement robust list cleaning practices, including email verification at the point of signup to prevent invalid addresses from entering your database.

Soft bounce management

Retries: Most ESPs automatically retry soft bounced emails over a set period. Monitor these retries to see if the temporary issue resolves. Review SMTP bounce codes for insights.
Thresholds: If an email repeatedly soft bounces (e.g., 3-5 times over several days), it's often best practice to remove it, as it may indicate a persistent issue or a de facto hard bounce. This also helps with over-quota mailboxes.

Email Service Providers typically manage bounces by categorizing them based on the received SMTP codes and implementing automated rules for suppression. Understanding how your ESP manages bounces is essential, as some might offer more granular control or automatic re-engagement flows for certain soft bounce types.

By actively managing both hard and soft bounces, you ensure that your email list remains clean and engaged. This proactive approach not only improves your immediate deliverability but also safeguards your long-term email marketing success. Reviewing your email bounce types regularly can provide valuable insights.

Impact on sender reputation

A high bounce rate, particularly from hard bounces, is a red flag for Internet Service Providers (ISPs) and can severely damage your sender reputation. ISPs monitor bounce rates as a key indicator of list quality and sender legitimacy. If they see a lot of hard bounces from your sending domain or IP address, they might start to view you as a spammer, leading to reduced inbox placement, or even complete blocklisting.

While soft bounces are temporary, a consistently high volume can also be problematic. Repeated soft bounces to the same addresses indicate an issue that isn't resolving, signaling to ISPs that your list might not be as engaged or well-maintained as it should be. This can still contribute to a decline in your email deliverability rate over time. It's not just about getting emails delivered, but about consistently reaching the inbox.

Bounce type	Definition	SMTP codes	Impact on sender reputation	Action required
Hard bounce	Permanent delivery failure, invalid or non-existent address.	5xx	High impact, immediate negative effect.	Remove immediately and permanently.
Soft bounce	Temporary delivery failure (e.g., full inbox, server down).	4xx	Lower impact, but repeated bounces can degrade reputation.	Retry a few times, then remove if persistent.

Maintaining a low bounce rate is a cornerstone of good email deliverability. By proactively identifying and addressing both hard and soft bounces, you protect your sender reputation, ensuring that your legitimate emails continue to reach your audience's inboxes. It’s an ongoing process that requires diligent monitoring and list hygiene practices. Neglecting it can lead to your emails being flagged as spam or even your domain being added to an email blacklist (or blocklist).

Views from the trenches

Best practices

Immediately suppress hard bounced email addresses from future sends.

Configure your ESP to retry soft bounces intelligently.

Analyze bounce patterns to identify underlying issues.

Validate email addresses at the point of collection.

Common pitfalls

Ignoring hard bounces and continuing to send to invalid addresses.

Failing to set up proper soft bounce thresholds.

Not differentiating between bounce types in your ESP.

Sending emails to purchased or old, unverified lists.

Expert tips

Actively monitor SMTP error codes for bounce diagnostics.

Implement double opt-in to reduce invalid sign-ups.

Segment lists to identify and isolate problematic subscribers.

Regularly cleanse your email lists of inactive addresses.

Marketer view

Marketer from Email Geeks says hard bounces occur when an address or domain does not exist, while soft bounces are due to temporary issues like a full inbox or auto-responders.

2019-01-10 - Email Geeks

Expert view

Expert from Email Geeks says the terms "hard bounce" and "soft bounce" are often ill-defined and used inconsistently across different contexts.

2019-01-10 - Email Geeks

Optimizing your email deliverability

Distinguishing between hard and soft email bounces is a foundational aspect of effective email deliverability and overall marketing strategy. Hard bounces demand immediate action due to their permanent nature and significant impact on sender reputation, while soft bounces require careful monitoring and a strategic retry approach.

By understanding these differences and implementing appropriate management strategies, you can maintain a clean, engaged email list, protect your sender reputation, and ensure that your email campaigns achieve their desired results. Proactive bounce management is an ongoing effort that pays dividends in improved inbox placement and campaign performance.