Bounced Emails

Why They Happen and How to Stop Them

Is your email bounce rate bringing your email campaigns down?

Learn about the email bounce types, how they hurt your performance, and what you can do to reduce email bounce rate now.

A man in a blue shirt looks at his Mac with a puzzled expression as he sees a bounced email with the message "not delivered" to john@company.com

Bounced emails aren’t just a nuisance. They can quickly sabotage all of your email marketing and cold outreach efforts.

But what causes an email to bounce?

Before we can explore the ‘why,’ let’s take a look at the two email bounce types to better understand the mechanics involved with sending an email message.

Email bounce types explained

There are two types of email bounces:

  • Soft bounce - Some temporary errors, such as a full mailbox or a timeout, cause soft bounce emails.
  • Hard bounceⓘThe inability to deliver an email message that’s caused by one or more permanent factors, such as an invalid email address. - Email hard bounces occur when there is a fixed error, such as attempting to send an email to an invalid email address or an invalid email domain.

Both email bounce types factor into your campaign’s overall bounce rate. It’s essential to monitor the bounce rates of your campaign for an unexpected or unusual rise in email bounces so that you can take action before it causes lasting damage to your domain or IP. Both soft and hard bounces can have a negative impact on your email marketing performance.

How do bounced emails impact email marketing performance?

Bounced emails are a powerful threat to your email marketing performance and ROI.

The reason for this is simple – you cannot generate engagement and conversions if your email doesn’t reach someone. However, the threat of high email bounce rates doesn’t stop at the campaign level. Continued bounced emails over time negatively impact your sender reputation/sender scoreⓘOften used interchangeably with Sender Reputation. It is a score used by internet service providers to communicate your reputation with others based on your historical email-sending habits and behaviors..

Your reputation is what internet and email service providers (ISPs & ESPs, respectively) use to determine the trustworthiness of an email sender. Email is used daily by bad actors looking to deceive readers with spammy content and phishing scams. Those types of senders care little for safe sending practices and will typically have domains and IP addresses with notoriously high email bounce rates. If your business also has a high email bounce rate, you immediately look suspicious regardless of what’s in your emails.

What happens when your sender reputation declines?

ISPs and ESPs recognize the emails coming from your domain and are more likely to filter your emails to spam automatically. They may also reject your emails outright, resulting in another bounced email. These negative signals continue to pile up and can make it difficult, if not impossible, for your emails to reliably reach your subscribers’ inboxes.

Your email service provider will also take notice. Most prominent ESPs like Mailchimp or Zoho enforce strict policies regarding their users and email bounce rates. If you bounce too many emails in one campaign or over time, they can and will suspend your business account.

To summarize how bounced emails impact email marketing performance:

  • Bounced emails can’t reach customersⓘThe act of successfully communicating with a customer or client via an email, text, or other mode of communication. to create engagement and sales
  • High email bounce rates damage your business sender reputation
  • As your reputation declines, your email deliverabilityⓘA sender’s ability to reach the recipient’s inbox with their outgoing emails. It may also describe the ratio of emails delivered to the inbox vs. those sent to spam or blocked by the receiving server. declines and puts your emails in spam
  • High email bounce rates also indicate other sending issues, which can cause your ESP to suspend your account

How to reduce email bounce rate

Understanding email bounce types gives us more information to identify why an email bounce may occur.

How to reduce soft bounce emails

Because temporary issues cause a soft bounce email, they’re often out of your control to prevent. It’s up to the mailbox owner or service provider to remedy bounce-causing issues like full mailboxes or connections. There’s also anti-spam technology called greylisting, which temporarily rejects emails from new senders for up to 15 minutes.

How to minimize soft bounces: keep emails lightweight, monitor link quality, keep a consistent sending schedule, and test your mail server.

Here’s how you can minimize the number of bounces that occur:

  • Keep your emails lightweight - Use small file sizes, particularly when adding images or attachments to an email. If the file is too big, it can cause timeout issues, resulting in a bounce.
  • Keep a consistent schedule, and avoid spamming - ESPs don’t like it if you send too many emails. Avoid soft bounce emails by sticking to a schedule that’s an optimal time for your readers.
  • Monitor link quality - Just like with your website, adding low-quality, spammy links to an email can hurt its deliverability. Be mindful of what you share with your readers.
  • Test your email server configuration - A well-configured email server with SMTP should automatically reattempt delivery if it receives a 4xx (temporary) error. This is critical for any temporary soft bounce causes, including common greylisting measures. If your server is configured properly, it should automatically re-attempt delivery after the greylist period passes, allowing it to reach the mailbox.

How to reduce hard bounces

Email hard bounces are a primary concern. If you’re experiencing a high volume of hard bounced emails, it’s typically caused by using invalid email addresses or an improperly configured mail server.

Here’s how you can reduce email bounce rates:

How to minimize hard bounces: clean your list, use double opt-in, use ,[object Object],, warm up your reputation, test your emails, use different mailing platforms, and avoid spam.
  • Clean your email list - Before you add an email address to your subscriber list, you must use an email validationⓘA process that determines if an email address uses valid syntax, exists on a given domain, and is configured to receive incoming email messages tool to clean it. This service ensures that the mailing address is valid and being used by an actual person. Remove any invalid email addresses from your list.
  • Use a double opt-in message - A double opt-in confirmation is now standard for email marketing. Whenever a user signs up with their email address, they must click a button in a confirmation email. Doing this verifies their address and their intent to subscribe.
  • Verify your domain with email authentication - Previously option, email authentication records like SPF, DKIM, and DMARC are now required by major mail providers like Google, Yahoo, and Microsoft. These records verify your business identity and ownership of the mailing domain, which helps your sender reputation and reduces the chance of email bounces.
  • Warm up your reputation - If you’re experiencing hard bounces, your sender reputation may already be damaged. Using an email warmupⓘA service that helps email senders improve their mailing domain and IP reputation by sending genuine email content to predetermined email addresses. Users will gradually increase the volume of emails sent while receiving positive engagement through an automated process. tool will automatically improve your sender score by sending regular emails that receive positive engagement signals like opens, clicks, and replies. Warming up your domain can help reverse existing damage while you implement all of these practices into your email strategy simultaneously.
  • Avoid sending spam - Be mindful of the emails you send to users. Be explicit about what’s in the email from your subject line, and avoid using spammy jargon that you’d typically see in a junk email (free stuff, asking for sensitive information, too good to be true deals, etc.).

    Additionally, spam also is characterized by your sending patterns. Send emails in a predictable pattern (set days and times) to avoid triggering any spam filters.
  • Use an inbox placement tester - If you have any doubts about your campaign hard bouncing, use a deliverability tester. This tool allows you to send your campaign to a list of privately owned addresses so that you can see if your campaign has been delivered and where it lands.
  • Use different mailing domains - If you’re looking to email new addresses, especially ones with risk like a catch-all, look into setting up different domains for your teams. Doing this is particularly valuable for anyone involved in cold email outreach, as you don’t want testing or experimentation to cause unnecessary damage to your primary business domain.

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    Using email bounce verification services

    Most of the time, an email hard bounce occurs because of an invalid email address. Whether this is a one-off occurrence or if hard bounces continue to happen due to irregular list cleaning, it’s never too late to get started.

    Here’s how you can incorporate bounce verification tools into your current workflow:

    1. Download your current mailing list from your CRM or mail provider.

    2. Upload your list to an email verification service. When you upload your list to ZeroBounce, it will automatically look for any duplicates on your list and remove them for you.

    The ZeroBounce email validation menu that includes an integration menu, file upload screen, and a 'Get Started' button for individual email validation.

    3. Allow the email verifier to verify and validate your email contacts. The duration will depend on the size of your list, and you’ll automatically be notified via email when the process completes.

    4. Download your new clean list of email addresses. You’ll also receive a report on which email addresses were invalid or high-risk to get a better sense of what type of data you’re acquiring.

    Bounce verification services are simple to use and an inexpensive way to drastically reduce email bounce rates. It’s a foundational tactic for an email sender or marketer that you can’t avoid doing. Some ESPs are so strict on email list cleaningⓘThe process of removing invalid and high-risk emails, such as spam traps or disposable emails, from an email list. Email list cleaning can be performed after gathering data via email validation. that they can immediately detect when bad data has been uploaded to their platform if they’ve seen those addresses or that list prior.

    In addition to direct uploads for cleaning, here are some additional ways you can leverage email bounce verification services:

    • Use an integration with your mail provider, CRM, or ESP - Some email verification platforms provide direct integrations with many marketing and sales tools you already use. ZeroBounce currently features 60+ integrations.

      With an integration enabled, you can sync your email address contacts with the tool and clean them automatically with little effort.
    • Use a real-time email verification APIⓘAn API, or application programming interface, that allows an email verification tool to connect and communicate with another software application. - The best way to reduce email bounce rates is to avoid gathering invalid data in the first place. By adding a real-time email verifier to your website, signup forms, landing pages, or checkout screens, the email verifier will automatically detect invalid or risky email addresses and block them. It’s also useful for detecting common errors like typos.

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    Frequently asked questions

    Bounced emails can be caused by temporary errors, such as a full mailbox or an email server timeout. They can also be caused by a permanent error, such as an invalid email address, which results in a hard bounce.

    There are two types of email bounces: a soft bounce and a hard bounce. A soft bounce email results in some temporary error, such as a full mailbox. An email hard bounces when there is a permanent error, like sending a message to an invalid email address.

    You can reduce email bounce rates by using best email practices, including the following: cleaning your email to remove invalid emails, authenticating your email domain, sending on a consistent schedule, and using a paid, trustworthy email service provider.

    While the impact of a soft bounce is not immediate, high bounce rates caused by continued soft bounces can negatively impact your sender reputation. Some errors, such as a full mailbox or a misconfigured server, are not under your control. However, you can mitigate soft bounces by cleaning your email list, improving your sender reputation, and ensuring that your mail server is configured properly.