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Find out what it means when an email bounces, the types of bounces

What does it mean when an email bounces?

What does it mean when an email bounces? The day-to-day life of those who work with email marketing campaigns is full of challenges and they come along with countless terms referring to all the most important aspects of this activity.

Delivery rates, subject and preheader, CTR, CTA, inbox placement and blacklist are just some of these terms.

But among the terms most feared by digital marketers is email bounce, which is when an email message is not successfully delivered to the email provider (ESP).

Therefore, understanding what it means when an email bounces is essential for any digital marketing strategy. In this article, you’ll learn about the concept of bounced emails, their types and how they can impact your email marketing campaigns.

What does it mean to say that an email has bounced?

To cut to the chase: an email is considered bounced when it cannot be delivered to the recipient for whatever reason, and returns to the sender.

When the email is sent directly by the sender via a webmail (from Yahoo!Mail, Gmail, Outlook.com, among others) or email software (MS Outlook, Thunderbird, for example), this message may bounce with a negative delivery message, such as “there is no user with this name”.

Email confirmation or denial messages are just a small part of the large universe of possible SMTP (Simple Mail Transfer Protocol) messages. This protocol communicates via codes between the sending and receiving servers. To make it easier for users to understand, these codes are accompanied by text messages.

Any failures in the delivery procedures of an email message are predicted by a type of SMTP code that explains the problems in order to allow them to be mitigated. For example, we can mention communication failures, timeouts, server instability, configuration problems, among many others.

In this article, we’re going to focus on SMTP messages that are referred to as soft bounce and hard bounce. From there, we will deepen our understanding of “what does it mean when an email bounces?”.

What does it mean when an email bounces? Soft bounce e Hard bounce

The reasons why an email address may bounce are quite diverse, as there are dozens of reasons that can interfere with the delivery and reception capacity between email servers (both on the part of the sending service and the service that receives and stores the message).

However, two reasons are more common and can be characterized more easily: hard bounces and soft bounces.

Soft bounce

When an email is bounced due to problems that are considered temporary, they are called soft bounces.

In other words, we have a soft bounce when the delivery problem can be resolved so that new delivery attempts can be made to the same email address in the future.

Therefore, the following can be listed as reasons for a soft bounce:

  • inbox full
  • connection problems
  • blockages due to blacklists
  • blocks due to spam complaints
  • blocks due to content or excesses (in general)
  • errors or absence of certain configurations or authentications

Then, once the problems identified have been resolved, these emails can usually be delivered in future attempts, as they are reasons that can be corrected.

Examples of soft bounce messages

Technically, soft bounces are messages such as:

  • Mailbox Full: 450 4.2.2 [email protected]: Recipient’s mailbox is full
  • Server Busy: 421 4.3.2 The recipient server is busy. Please try again later.
  • Message Size Exceeds: 452 4.3.1 [email protected]: Message size exceeds fixed maximum message size
  • DNS Failure: 450 4.1.2 [email protected]: DNS failure. Try again later.
  • Mailbox Temporarily Unavailable: 451 4.2.1 [email protected]: Mailbox temporarily unavailable. Try again later.
  • Daily Quota Exceeded: 451 4.7.1 Daily sending quota exceeded. Please try again tomorrow.
  • Blocked by Spam Filter: 451 4.7.0 Message blocked due to content resembling spam. Please review your message and try again.
  • IP Blacklisted: 451 4.7.1 [email protected]: Recipient address rejected: Your IP address is blacklisted.
  • Content Filtered: 451 4.3.0 Message temporarily rejected by content filter. Please modify your message and try again.
  • Spam Score Too High: 451 4.7.1 Message rejected due to high spam score. Please review your message content.
  • Recipient Domain Blacklisted: 451 4.7.1 [email protected]: Recipient domain blacklisted. Please contact the recipient via other means.

Hard bounce

When an email is not delivered for reasons that are considered to be of a permanent, immutable nature, they are considered hard bounces.

In other words: we have a soft bounce when an email address will never be able to receive messages, either now or in the future.

In this way, emails that are considered hard bounces:

  • Invalid (incorrect syntax)
  • Do not exist (email account does not exist)
  • Domains unable to receive emails (no MX set up)

So beware: as the problems of hard bounces cannot be solved, you shouldn’t attempt to send messages to these addresses again, as this could have a negative impact on your email marketing.

Examples of hard bounce messages

Technically, hard bounces are messages such as:

  • User unknown: 550 5.1.1 [email protected]: Recipient address rejected: User unknown in virtual mailbox table.
  • Mailbox Not Found: 550 5.1.1 [email protected]: Recipient address does not exist.
  • Domain Not Found: 550 5.1.2 [email protected]: Host or domain name not found. Name service error for domain domain.com: Host not found
  • Permanent Failure: 550 5.1.1 The email account that you tried to reach does not exist. Please try double-checking the recipient’s email address for typos or unnecessary spaces.
  • Mailbox Disabled: 550 5.2.1 [email protected]: Recipient address rejected: Mailbox has been disabled
  • Rejected Address: 550 5.1.1 [email protected]: Recipient address rejected: Address not found
  • Account Disabled: 550 5.2.1 [email protected]: Mailbox disabled for this recipient
  • Invalid Email Address: 550 5.1.3 [email protected]: Invalid email address

Impact of bounced emails on delivery rates

High bounce rates aren’t just a practical problem, with messages that simply haven’t been delivered. On the contrary.

Bounced emails can, in addition to damaging your reputation as a sender and negatively affecting your delivery rates, cause further damage to your sales results, in a strong cascading effect.

Email service providers constantly monitor these rates and can trigger blocking mechanisms and other types of punishment, even considering you a spammer.

What happens when an email is soft bounced?

Let’s start from the beginning: what does it mean when an email bounces? Well, it can be seen as a symptom of a problem or as the result of a problem. Let’s explain further:

Soft bounce can represent a symptom of problems. An example of this is if the soft bounce happens because of configuration failures or even connection timeouts. This is a symptom of negligence with regard to the technical quality of email sending, which will be flagged by the providers.

However, when the soft bounce happens because of a block due to too many messages being sent or because the sending domain is already on a blacklist, then it is possible to say that this soft bounce is a consequence or result of previous unresolved problems, such as spam mailings, high complaint rates, increases in the daily volume of mailings, among others.

So, we can say that the soft bounce spells out problems that must be corrected before the next sends, otherwise more and more messages will be blocked and more anti-spam actions (such as reports to blacklists) will be taken.

Secondly: with valid emails no longer being delivered due to technical or behavioral issues, fewer recipients will be able to contact you.

Thirdly: delivered messages may be allocated to the spam folder instead of the inbox.

Summary of the problems caused by soft bounces:

This is a list of some of the problems caused by soft bounces:

  • Blocking
  • Penalty actions (sending information to blacklists)
  • Unblocked messages may be delivered to the spam folder (low deliverability)
  • Reduced read, click and conversion rates

What happens when an email is hard bounced?

In the case of hard bounces, the problems are more serious. You’re sending email campaigns to addresses that don’t exist (for various reasons).

So, when an ISP bounces emails, it starts a count that takes into account the ratio of valid to invalid emails in your campaign. When this ratio gets close to 3%, the email service provider (ESP) will take more severe measures.

Firstly, they will block your campaign for too many hard bounces. In other words: hard bounce messages trigger a soft bounce process.

In other words, your valid emails stop being delivered because of invalid emails.
The second measure is to lower your reputation score (senderscore) and, in the event of recurrence, report your domain and IP addresses to blacklists.

In more serious cases, the email provider may simply stop delivering emails altogether instead of blocking them, which would be disastrous.

Summary of the problems caused by hard bounces:

This is a list of some of the problems caused by hard bounces:

  • Blocking
  • Penalty actions (sending information to blacklists)
  • Unblocked messages may be delivered to the spam folder (low deliverability)
  • Interruption of message reception
  • Drastic reduction in read rates, clicks and conversions

How to reduce bounce rates

How to reduce soft bounces

To mitigate soft bounces, it is necessary to identify the return messages in order to know which problems need to be resolved.

Then take the necessary action. For example:

  • Problems of excess: if you are sending too many emails, reduce your daily mailing list. If the email is too heavy, revise the html code to make the message lighter.
  • Blacklisted IP addresses or domains: have the record removed from any blacklists where it is listed. You can do this at https://blacklistalert.org/
  • Content blocks: review the content of the email for inappropriate words, shortened hyperlinks or hyperlinks pointing to malicious sites, etc.
  • Technical blocks: review SPF, DKIM and DMARC settings, experiment with sending campaigns at different times, etc.

How to reduce hard bounces

To reduce or even put an end to hard bounces, it is possible to do this BEFORE sending out email marketing campaigns, avoiding unnecessary penalties.

So, take the following measures:

  • Pay attention to email collection points: forms on landing pages, points of sale, among others, should have email syntax criticism, as well as Captcha protection.
  • Confirm the email: using double opt-in techniques helps prevent invalid emails from being registered, even in unintentional cases such as typos.
  • Use an email verification platform: verification platforms can remove invalid emails (among others) from mailing lists, as well as perform real-time email verification on registration forms.
  • Remove unengaged emails: these email addresses can be deactivated after a period of inactivity.

SafetyMails offers an email verification service, removing invalid emails, spam traps, disposable/temporary emails and other types of emails that are harmful to email marketing. It also has an API for real-time verification of forms.


Now you know what it means when an email bounces. Understanding and managing bounced emails is crucial to the success of your email marketing campaigns.

By following the practices we recommend, you can minimize the occurrence of bounces and ensure better delivery of your messages.


What is an email bounce?

An email bounce occurs when the message sent cannot be delivered to the recipient and returns to the sender. This can happen for various reasons, such as temporary connection problems, full inboxes or even invalid email addresses. Understanding bounced emails is fundamental to optimizing email marketing strategies and ensuring that messages reach their intended recipients.

What are the types of email bounce?

There are two main types of bounce: soft bounce and hard bounce. Soft bounce happens when the delivery problem is temporary, such as a full inbox or content blocks that can be resolved, allowing future delivery attempts. Hard bounce occurs when delivery is impossible for permanent reasons, such as invalid or non-existent email addresses, which will never be able to receive emails.

What causes a soft bounce?

A soft bounce can be caused by a number of temporary factors. These include full inboxes, connection problems with the recipient’s server, content blocks, emails filtered for looking like spam or too many sent every day. These problems can usually be corrected, allowing the email to be delivered on a future attempt.

What is a hard bounce and how can it be avoided?

A hard bounce occurs when an email cannot be delivered due to permanent problems, such as an invalid or non-existent email address. To avoid hard bounces, it’s important to keep a clean and up-to-date email list, use email confirmation methods (such as double opt-in) and verify email addresses before sending marketing campaigns.

How do bounces affect email marketing campaigns?

High bounce rates, both soft and hard, can damage the sender’s reputation and decrease the delivery rates of future emails. Email service providers monitor these rates and can block or limit the sending of emails from senders with a lot of bounces. In addition, campaigns with too many bounces can have their messages filtered into spam folders, reducing the effectiveness of marketing communications and negatively impacting conversion and engagement results.