Email Security Intelligence - Hard vs Soft Email Bounces: The Differences and How to Avoid Them

Have you ever sent an email only to receive a notification saying it wasn't delivered? This frustrating outcome is called an email bounce, which could happen for a number of reasons.

In this article, we'll detail the differences between hard and soft email bounces, their causes, and how to improve your sender reputation while maintaining email protection for your company and recipients. 

What are Email Bounces?

Emails are considered “bounced” when delivery fails. You'll often receive an auto-response reply advising you the email has yet to reach its intended recipient. There are two email bounce types: a hard and a soft email bounce.

Hard Email Bounce

Hard email bounces indicate a permanent delivery failure due to sending an email to an address that either doesn't exist (perhaps a result of misspelling) or isn't accepting incoming mail. An Email Service Provider (ESP) may have flagged your emails as spam or potential fraud, and these bounces are recorded on your account every time they occur. A distributed spam attack or email bomb can cause a hard email bounce.

Soft Email Bounce

Soft email bounces are Less severe than hard email bounces usually indicate a temporary issue preventing the message from being sent. Reasons for a soft bounce include a recipient's inbox being too full to accept incoming mail, the message being too large to be accepted, or an ESP server timing out. Since these bounces are only for a little time, ESPs will attempt a re-delivery, but if the message continues not to be sent, it will be redefined as a hard bounce, and no further attempts will be made to ensure email protection. 

What are the Differences between Hard and Soft Email Bounces?

A soft bounce is a temporary issue.

As mentioned, soft email bounces are fixable issues, whereas hard email bounces are permanent problems. 

A soft bounce is fixable when you diagnose a problem.

Soft email bounce conflicts can be rectified by the recipient emptying their inbox or adjusting other issues in order for the message to be received on a later attempt.

A hard bounce is a permanent failure you cannot fix.

Unfortunately, hard email bounces are not fixable, and little can be done to care for them.

What is an Acceptable Email Bounce Rate?

One of your company’s service goals should be to ensure that your email bounce rate stays below 2%, as anything higher is considered an issue for your sector. The rate can vary, but you should be concerned the second they go above the ones predicted for your company. For example, in e-commerce, your average bounce rate should be around 0.19% for hard bounces and 0.26% for soft bounces. To avoid having higher rates, make sure to go through old, outdated subscriber lists to make sure that you maintain as many customers as possible. This will ensure email protection on your end and prevent email security issues on the recipient’s.

What Are the Causes of Soft Email Bounces?

Full inbox

If there is no room left in the recipient's inbox to receive the message, or the message is too large for the space left, any incoming mail will be rejected.

Automated content blocker

When the recipient (or the ESP) has an automated content blocker, certain types or sizes of content may be automatically rejected. Take a look at the information in your email and edit it so that it will be accepted.

Incorrect recipient address

This is a fixable issue if it resulted from a mistyped address, but if you wrote it correctly and it still won’t send, there is not much that can be done.

Nonexistent domain name

If you send to a nonexistent domain name, reach out to the field for assistance so that you don’t experience even more errors within the server.

Outdated email list

Ensure you update your email lists regularly and have subscribers verify their information to guarantee that you only send messages to active accounts.

Domain Name System (DNS) failure

This could simply result from the recipient's email server being down or typos causing problems for the server.

Message too large

The recipient or their ESP may have a limit on the size or quantity of attachments they receive, which could prevent a message from being delivered.

Limit Reached

Some ESPs limit how many emails can be sent or received daily. If you encounter this issue with your ESP, continue switching.


Sometimes, you may receive a notification that a message soft bounced even though the email went through to the recipient. This could happen when recipients have auto-reply set up since they are away from their desks or on holiday. 

What Are the Causes of Hard Email Bounces?

The recipient doesn't exist

If the intended recipient has deleted their account or given you a false email address, your ESP will classify it as a hard bounce and make no further attempts to deliver the message. 

Email blocked by server

With cybercrime on the rise, there are increased security measures put in place by ESPs, companies, and individuals. These measures, which include SPF, DKIM & DMARC email authentication procedures, ensure that you are who you say you are. However, if a server finds your email suspicious, it will be processed as a hard bounce.

Mailbox is full

A full mailbox is only a soft bounce until repeated delivery attempts continue to fail.

Challenge-response error

This happens when recipients or ESPs set up an additional firewall to authenticate email senders. You'll be asked to verify your identity if it's your first time contacting the recipient, and failure to do so will cause the message to be processed as a hard bounce.

Poor server reputation

Messages may be rejected if a server constantly receives spam messages from you or hackers who hijacked the system. You can prevent such issues by protecting your secure email and servers with a comprehensive cloud email security software solution.

Email Sender Reputation Explained

If you send many emails, Internet Service Providers (ISPs) assign you an email sender reputation score reflecting your organization’s trustworthiness. Higher scores will have few significant email security issues, while low scores will result in many hard email bounces.

How do you protect your sending reputation?

  • Check your email lists regularly and determine which ones need updating or never respond. High bounce rates will lower your reputation score, so refining your list will keep bounces from happening as frequently. 
  • Ensure subscribers can access the opt-in policy to verify email addresses and highlight their agreement to receive email. 
  • Closely monitor your email delivery metrics to identify non-responders and potentially invalid addresses. 
  • Differentiate between hard and soft bounces to determine what is a solvable issue and an unfixable problem.
  • If you have a new account, build up sending volumes gradually. Security algorithms may flag your emails as spam if you immediately launch extensive campaigns. 

How is domain reputation calculated?

Many providers guard their algorithms, but your domain reputation score is similar to a credit score with a 1-100 scale. Here are some factors to consider when calculating your company’s score: open rate, reply rate, reported spam rate, read rate, deleted without reading rate, identified as “not spam” rate, spam placement rate, click rate, and hard bounce rate. 

How to Improve Your Sender Score

If your score is low or has dropped, focus on the following:

  • Regularly carry out housekeeping on your email list and remove subscribers who never engage.
  • Consider using email verification tools (or a double opt-in confirmation) to avoid incorrect addresses. 
  • Build your subscriber emails organically and avoid buying lists.
  • Stick to stated schedules and frequency.
  • Research spam trigger words that may be flagged.
  • Keep content relevant so it isn't faded.
  • Use advanced email threat security options to ensure email protection.
  • Remember the “How do you protect your sending reputation?” suggestions.

Tips on How to Avoid Email Bounces

Follow the best practices for email security and avoid email bounces to lower your email bounce rates. Here are a few things to prevent or reduce bounce rates:

Don't send marketing emails from a free mail service

Using Gmail, Yahoo, or any other free email software might result in your messages going through content and email filters, email protection features, and anti-spam services, all of which can harm your deliverability. There are various ways to send messages.

Suppose you have a website or operate online. In that case, your web host should also provide email addresses that give you domain-based email accounts that will improve your reputation and deliverability. 

Make sure your domain is authenticated

Any Software as a Service (Saas) app requires domain reputation protection through authentication measures, verifying any email as it travels from the sender to the receiver, reducing the chances of spamming.

Authentication has four basic steps:

  1. A business/organization establishes an email security policy for its outgoing emails.
  2. All email senders configure their email settings to comply with the policy.
  3. Emails received from the domain are then checked against the established rules.
  4. The receiving server will then act depending on whether the email has been authenticated.

There are three main authentication tools you can use to ensure your secure emails are delivered without a problem:

  • Sender Policy Framework (SPF) verifies the sender of an email and fights against impersonation and other types of phishing attacks. 
  • DKIM adds a digital signature to an outgoing email, allowing servers to verify that the message hasn't been altered or tampered with.
  • DMARC email authentication combines SPF and DKIM and allows you to check if the "from" address in the email matches the listed email address, permitting you to flag emails as suspicious.

If your domain is authenticated, your emails should transfer more smoothly through spam filters and other email security measures to be delivered safely to the recipient's inbox. 

Keep Learning About Emailing Safety and Email Protection

Email remains a strong and effective method of business communication; however, many spam emails are sent daily. In December 2021, 45.37% of emails were classified as spam. You must consider your email accounts’ and servers' security. A robust email security software solution can guard against malicious hackers and cybercriminals attempting to hijack your accounts, which could damage your company’s reputation severely. 

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