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---
title: "Email Encyclopedia: What is a Soft Bounce"
date: 2025-07-24
artist: Yuanshu
summary: "A soft bounce occurs when an email fails to deliver due to temporary issues, such as full mailboxes or unavailable servers. Understanding this mechanism helps improve email marketing effectiveness."
tags: ["Email Encyclopedia", "Alibaba Mail"]
keywords: ["Electronic Mail, Soft Bounce, Hard Bounce, Email Marketing, SMTP Protocol, Bounce Processing, Sender Reputation, Email Deliverability, Mail Server, Bounce Reasons"]
description: "A soft bounce occurs when an email fails to deliver due to temporary issues, such as full mailboxes or unavailable servers. Understanding this mechanism helps improve email marketing effectiveness."
---
![Alibaba Mail More Products and Services](https://gw.alicdn.com/imgextra/i1/O1CN01pbjbx71gOy1QtUSWO_!!6000000004133-2-tps-800-240.png) 


**Soft Bounce** is a common term in electronic mail communication, referring to situations where an email fails to be successfully delivered to the recipient's mailbox due to temporary issues. Unlike hard bounces, soft bounces are not caused by permanent problems such as invalid email addresses or non-existent domains, but rather by temporary technical obstacles or limitations, such as temporarily unavailable mail servers, full recipient mailboxes, or emails exceeding size limits.

In email marketing and large-scale email delivery systems, soft bounces are an important metric for evaluating email delivery success rates and email list quality.

## Definition of Soft Bounce

A soft bounce refers to when a mail server encounters **temporary obstacles** while attempting to deliver an email, preventing immediate delivery, but the system will still attempt to redeliver within a certain timeframe. This type of bounce is usually not immediately removed from the mail queue but is temporarily retained for subsequent delivery attempts.

In contrast, a **Hard Bounce** refers to **permanent delivery failures** due to invalid email addresses, non-existent domains, or non-existent accounts. Hard bounces typically don't receive further delivery attempts and should be removed from mailing lists to avoid affecting the overall quality of email sending.

## Common Causes of Soft Bounces

Soft bounces can have various causes, usually related to the recipient's mail server or account status. Here are some common reasons for soft bounces:

1. **Mailbox is Full**  
   The recipient's mailbox storage space is full and cannot receive new emails.

2. **Mail Server Temporarily Unavailable**  
   The recipient's mail server is temporarily unable to receive emails due to maintenance, failure, or network issues.

3. **Email Size Exceeds Limit**  
   The email content (especially attachments) exceeds the maximum email size limit allowed by the recipient's server.

4. **Mail Queue is Full**  
   The recipient's mail server queue is full and temporarily unable to receive new emails.

5. **Anti-spam Mechanism Triggered**  
   The recipient's anti-spam system is suspicious of the email content and temporarily refuses to receive it.

6. **Temporary DNS Resolution Issues**  
   The sender encounters temporary DNS resolution failures when trying to resolve the recipient domain's MX records.

7. **Connection or Rate Limitations**  
   The recipient's server has limited the connection frequency from a certain IP address or domain, causing the email to be temporarily rejected.

## Soft Bounce Processing Mechanism

In email systems, soft bounces are usually recorded and temporarily retained by the mail server. The system will attempt to redeliver the email multiple times within a set period according to the configured retry strategy. If the problem remains unresolved during the retry cycle, the email may be marked as a hard bounce and ultimately abandoned.

### Retry Strategy

Different mail servers and Email Service Providers (ESPs) may employ different retry strategies, but they typically include the following aspects:

- **Wait a period after the first failed attempt** (e.g., 30 minutes)
- **Subsequently retry at regular intervals** (e.g., once every hour)
- **Maximum number of attempts** (e.g., up to 16 attempts)
- **Total retry time limit** (e.g., up to 48 hours)

If the issue is resolved during the retry cycle, the email will be successfully delivered; otherwise, it will be marked as a hard bounce and removed from the queue.

## Impact of Soft Bounces on Email Marketing

In the field of email marketing, managing soft bounces is crucial for maintaining the health of email lists and improving email deliverability. Here are the potential impacts of soft bounces on email marketing:

### 1. Affects Email Deliverability

Although soft bounces are temporary, a high soft bounce rate can lead to a decrease in overall email deliverability, affecting the effectiveness of marketing campaigns.

### 2. Consumes Resources and Bandwidth

Mail servers need to constantly attempt to resend emails when processing soft bounces, consuming additional server resources and network bandwidth.

### 3. Impacts Sender Reputation

Frequent soft bounces may be viewed as poor behavior by Email Service Providers (like Gmail, Outlook, etc.), affecting the sender's **Sender Reputation**, which in turn affects the delivery success rate of future emails.

### 4. Requires Regular Email List Cleaning

To reduce the occurrence of soft bounces, email marketers need to regularly clean their email lists, removing mailbox addresses that have been unresponsive for a long time or frequently bounce.

## How to Reduce Soft Bounces

To lower soft bounce rates and improve email deliverability, email senders can take the following measures:

### 1. Use High-Quality Email Lists

Ensure that the email addresses in your list are genuine and valid, avoiding purchased or unverified email addresses.

### 2. Regularly Clean Email Lists

Periodically remove addresses that haven't opened emails for a long time or frequently bounce, maintaining the activity and health of your email list.

### 3. Follow Email Sending Best Practices

Include using clear subjects, avoiding spam keywords, providing unsubscribe links, etc., to reduce the likelihood of being intercepted by anti-spam systems.

### 4. Control Email Content Size

Avoid sending large attachments. It's recommended to use cloud storage links instead of direct attachments to reduce soft bounces caused by exceeding email size limits.

### 5. Use Professional Email Service Providers (ESPs)

Choosing reputable, technically mature email service providers helps improve email deliverability and reduce soft bounces.

### 6. Monitor Bounce Logs

Regularly review bounce reports provided by mail servers or ESPs, analyze the specific causes of soft bounces, and take appropriate measures for optimization.

## Differences Between Soft Bounces and Hard Bounces

| Characteristic | Soft Bounce | Hard Bounce |
|------|--------|--------|
| Nature | Temporary issue | Permanent issue |
| Retry Attempt | Yes, system will try to resend | No, email delivery is abandoned |
| Common Causes | Full mailbox, server unavailable, oversized email | Non-existent mailbox, invalid domain, deleted account |
| Impact on Email List | Acceptable, but needs monitoring | Should be removed immediately |
| Impact on Sender Reputation | Minimal | Significant |

## Technical Implementation Principles of Soft Bounces

In the SMTP (Simple Mail Transfer Protocol), mail servers judge whether an email has been successfully delivered based on the returned status code when attempting delivery. When encountering temporary errors, the server returns a **4xx** class status code (such as 450, 452, etc.), indicating a soft bounce; while **5xx** class status codes (such as 550, 553, etc.) indicate hard bounces.

For example:

- **450 4.2.1 Mailbox unavailable**: Indicates the mailbox is temporarily unavailable, classified as a soft bounce.
- **550 5.1.1 User unknown**: Indicates the user does not exist, classified as a hard bounce.

Mail servers decide whether to retry sending emails based on these status codes.

## Conclusion

Soft bounces are an inevitable phenomenon in electronic mail communication. Understanding the causes, processing mechanisms, and impacts of soft bounces on email marketing helps email senders better manage email lists, optimize email sending strategies, and improve overall email deliverability. By regularly maintaining email lists, following best practices, and using professional email service platforms, the occurrence of soft bounces can be effectively reduced, thereby enhancing electronic mail communication efficiency and user experience.

## References

1. [RFC 5321 - Simple Mail Transfer Protocol](https://tools.ietf.org/html/rfc5321)
2. [Mail Delivery Status Codes](https://www.ietf.org/rfc/rfc3463.txt)
3. [What is a Soft Bounce? - Mailchimp Glossary](https://mailchimp.com/help/glossary/soft-bounce/)
4. [Bounce Types and Diagnostics - SendGrid Documentation](https://docs.sendgrid.com/for-developers/smtp-api/bounce-types-and-diagnostics)
5. [Email Bounce Management - Campaign Monitor](https://www.campaignmonitor.com/resources/guides/email-bounce-management/)