Why Do Emails Bounce? Understand Email Bounces and Learn to Fix Them

why do emails bounce cover image with a tennis ball

An email “bounces” when it can’t reach the recipient’s address and returns an error message to the sender. 

While it’s easy to think there might be a problem with your setup, emails bounce for a variety of reasons. So, if you just can’t figure out why some of your messages aren’t getting through, the reason might be something that’s out of your control. 

Keep reading as we explore the common causes of email bounces and a few straightforward steps you can take to prevent them.

Email Bounces Come in Two Flavors

When an email bounces, it’s usually categorized as either a hard bounce or a soft bounce. Each type tells you something different about why your email didn’t make it through.

Hard Bounces occur when the email is permanently rejected. This might be because the email address you sent it to doesn’t exist anymore, or there’s a mistake in the address. Think of it as sending a letter to a house that’s been torn down.

Soft Bounces are temporary problems. These can happen if the recipient’s mailbox is full, their server is down, or the email is too large. Imagine knocking on someone’s door, and they can’t answer right now because they’re busy; you might try again later.

Email Bounces Have Three Main Causes 

Emails don’t just bounce because there’s something wrong with the sender’s configuration. There are three main categories of issues that can cause an email to bounce back:

Recipient Issues

These are problems related to the recipient’s email address or mailbox that prevent your email from being delivered. Issues in this category typically revolve around the status or configuration of the recipient’s email account.

Sender Issues

These involve challenges stemming from your own email setup or behavior. They include your list management practices to how other entities on the internet view your server. 

Server Issues

These are related to technical problems with the email servers. They can occur on either side — the sender’s or the recipient’s — and often involve communication errors or configuration mismatches between different email systems.

CategoryCommon Causes 
Recipient issues ‣ Full mailbox
‣ Email address no longer exists
‣ Email blocked by recipient’s filters
‣ Mailbox size limits exceeded on the recipient server
Sender issues‣ Incorrect email address
‣ Poor sender reputation 
‣ Lack of authentication (SPF, DKIM, DMARC failures)
‣ Sending too many emails too quickly (spam triggers)
Server issues ‣ Email server is temporarily unavailable
‣ Network congestion affecting data transmission
‣ Aggressive firewall or security settings blocking the email
Three categories of causes for bounced emails 

With that overview, let’s consider the individual causes of why emails bounce. 

Recipient Issues Causing Email Bounces 

These issues are usually out of the sender’s control and occur on the recipient side. Most of the solutions involve waiting or removing the recipient from your list. Let’s go over the top four ones. 

Cause 1: Full Mailbox

Email services set a limit on the amount of data each account can hold. When the recipient’s storage is at capacity, emails bounce back because there’s no room for new messages. It’s similar to a mailbox at a house being stuffed full so that no more letters can fit.

The server then sends a notification back to the sender, indicating that the email couldn’t be delivered due to a lack of space.

Potential solutions:

  1. Notify the recipient: If possible, contact the recipient through another communication method, like a phone call or text, to let them know their mailbox is full and that they’re missing out on incoming emails.
  2. Resend later: Sometimes, you might just need to wait and resend the email after giving the recipient some time to clear out their mailbox.
  3. Remove from your list: If the recipient is on your email marketing list, we recommend removing them because they probably don’t use that address frequently enough. 

Cause 2: Email Address No Longer Exists

Users or email providers delete or deactivate accounts for many reasons. The user may switch to a different email service, or the account could be flagged for security reasons. Businesses also close email accounts when employees leave the company.

If you send an email to an address that no longer exists, the email server on the receiving end can’t find an account to deliver your message to. So, the server sends the email back to you with a message explaining that the address couldn’t be found.

Potential Solutions:

  1. Double-check the email address: Make sure you haven’t made any typos in the email address. 
  2. Update your email list: If the email address is indeed no longer active, remove it from your email list. Consistently sending to invalid addresses can send a negative signal to ISPs, which might hurt your ability to deliver emails in the future.

Cause 3: Email Blocked by Recipient’s Filters

Email filters scan messages for specific criteria such as keywords, the sender’s reputation, or suspicious email attachments. If an email meets the filter’s criteria for being unwanted or potentially harmful, it automatically gets blocked or sent to the spam folder.

If a filter blocks an incoming email, the sender will receive a message explaining that the email couldn’t be delivered due to a filter restriction. The level of detail in the message varies from one service provider to another. 

Potential solutions: 

  1. Ask to be whitelisted: Contact your recipient using another channel and ask them to whitelist your email address or domain to make sure your emails always reach their inbox.
  2. Follow best practices: Avoid using spammy words, excessive links, or large attachments that can trigger filters.
  3. Engage recipient interactions: Encourage your recipients to interact with your emails, like replying or clicking on a safe link. Positive interactions can improve your sender reputation and reduce the chances of being blocked by filters.

Cause 4: Mailbox Limits Exceeded on Recipient Server

Email services set limits on the number of emails an inbox can receive in a specific period. These limits help prevent spam attacks, which could overwhelm both the recipient’s inbox and the email server. They also keep the service reliable and efficient for everyone.

Typically, if a mailbox hits its limit, it’s just a temporary block, and the cap usually resets within an hour. After that, the server will start accepting emails again.

Potential solutions:

When this happens, your only option is to defer the email and try again in a few hours to make sure the limit has reset. If the issue persists, the recipient inbox is probably compromised. So, you should remove that address from your list to keep your email list pristine. 

Sender Issues Causing Email Bounces

These issues are caused by a problem on the sender side. In most cases, the email address is misspelled or invalid. Extremely spammy behavior can also lead to bounced emails. 

Cause 1: Incorrect Email Address

One of the most common reasons for email bounces is simply typing the wrong email address. Maybe there’s a typo, an extra letter, or a missed part of the address that leads to the email going nowhere. Fortunately, it’s also one of the easiest problems to fix.

Potential solutions: 

  1. Use double opt-in: Double opt-in involves sending a confirmation email to new subscribers asking them to verify their email addresses. This step ensures the address is correct and the subscriber genuinely wants to receive emails from you. 
  2. Clean your list: If you find that an email address is invalid, remove it from your list. Keeping your list clean reduces bounce rates and improves the effectiveness of your email campaigns.

Cause 2: Poor Sender Reputation

Mailbox providers use various factors to determine whether you’re a spam sender. If they suspect your sender reputation, they might decide to bounce your emails.

For example, if emails from your domain receive a high number of spam complaints, ISPs may limit or even block all messages coming from your domain or email IP address.

Potential solutions: 

  • Remove complaining users: If a user marks one of your emails as spam, the standard practice is to remove them from your list immediately. Your mass email software should have the feature to do this automatically. 
  • Improve email engagement: Send emails that your subscribers want to open and engage with. Regularly update your content to keep it relevant and interesting. More opens and clicks can boost your sender reputation.  
  • Segment your lists: Send emails to segments of your list that are most likely to engage. Don’t blast the same email to your entire list, especially if some parts of your list have lower engagement rates.

Cause 3: Lack of Domain Authentication 

Domain authentication proves that an email comes from the source it claims to come from. ISPs are more likely to reject messages from unauthenticated domains because they resemble spam. 

For example, this is the error message Gmail uses when it bounces an email due to lack of authentication: 

This message does not pass authentication checks (SPF and DKIM both do not pass). To best protect our users from spam, the message has been blocked.

What’s more, as of February 2024, major email providers like Google and Yahoo have updated their mass email guidelines. Now, they require everyone sending mass emails to use domain authentication.

Potential solutions: 

The solution here is to implement domain authentication protocols, which basically add DNS records to your domain. These are the three we recommend: 

  • SPF (Sender Policy Framework): This protocol verifies your email by associating the sender’s domain with a list of IP addresses authorized to send mail from that domain.
  • DKIM (DomainKeys Identified Mail): DKIM adds a digital signature to your emails. This signature helps receiving email systems verify that the email hasn’t been tampered with during transit and that the sender is legitimate.
  • DMARC (Domain-based Message Authentication, Reporting & Conformance): DMARC uses SPF and DKIM to provide a way for email senders and receivers to improve and monitor protection from spam. It also gives instructions to receiving servers on what to do if neither SPF nor DKIM passes authentication.

Cause 4: Sending Too Many Emails Too Quickly 

ISPs have limits on how many emails they can handle from one sender in a given period. If you send out a lot of emails very quickly, it might look like spam, and the ISP might bounce your emails back. This situation is known as email throttling.

The number of emails you can send depends on your IP reputation, spam complaints, email bounce rates, and subscriber engagement. For example, a new IP address might have a lower daily volume threshold until it establishes a good sending reputation. 

While platforms like Gmail and Outlook might limit user accounts to 300-500 emails per day, an email marketing platform with a pristine IP reputation could handle millions of messages per day.

Potential solutions: 

  • Warm up your IP address: Warming up an IP means gradually increasing the volume of emails you send over a period to build a good reputation with ISPs. Start with a smaller number of high-quality, engaging emails and slowly ramp up the volume.
  • Monitor your send rates: Keep an eye on how fast you’re sending emails. Adjust the pace based on the feedback from ISPs and bounce messages you receive to stay below the throttling limits.
  • Use a reputable email marketing service: These services often have established reputations and the capability to send large volumes of email without triggering spam filters. They can manage send rates and help you avoid throttling issues.

Server Issues Causing Email Bounces 

This category of reasons for email bounces relates to the servers that transfer your message across the internet from your device to the recipient’s. Each email typically makes a few hops to reach its destination, and any one of these hops could cause a bounce.

Cause 1: Email Server Temporarily Unavailable

Sometimes, the server that receives your email might be down temporarily. This can happen because of routine maintenance, unexpected outages, or technical glitches. When this server isn’t working, it can’t accept any emails, causing them to bounce back.

Potential solutions: 

Often, the best approach is simply to wait a bit and then try resending your email. Server issues are usually resolved quickly.

Cause 2: Network Congestion 

Network congestion happens when too many users or too much data tries to travel across the internet at the same time. Think of it like rush hour traffic on a highway. When this happens with your emails, they might time out or bounce back because they can’t reach the recipient’s server in time.

Potential solutions: 

Keep an eye on when you experience these issues most often and plan your important communications accordingly. Many email platforms also let you schedule your emails, so you can set them to go out at times when network congestion is less likely.

Cause 3: Aggressive Firewall or Security Settings 

Sometimes, servers are set up to reject all or some incoming emails. This could be due to a mistake in how the firewall or security settings are configured, or it might be intentional to protect networks from potential attacks. 

This situation doesn’t usually affect your sender reputation and it’ll resolve on its own. If the issue persists, you should remove that contact from your email list. Another option is to contact the recipient and ask them to whitelist your address

SMTP Error Codes Point to the Cause 

The Simple Mail Transfer Protocol is a set of standards for sending and routing emails between servers on the internet. Once an email arrives at a server, the server issues a three-digit code to indicate the status of the email delivery attempt.

Here’s a breakdown of the main SMTP error codes: 

  • 2xx Responses (Success codes): The server received the message successfully. 
  • 4xx Responses (Transient failure codes): These are temporary errors and mean that the email wasn’t delivered. In other words, these indicate soft bounces. 
  • 5xx Responses (Permanent failure codes): These indicate a hard bounce, meaning that the server has rejected the email.

Soft Bounce: Temporary delivery failure

Bounce CodeDescriptionAction
421Your message was temporarily deferred by the recipient server.The recipient server might be overloaded. Try again later. Consider sending smaller emails or attachments.
422The mail server encountered a temporary failure.This is usually caused by congestion. Retry sending later. 
451Requested action aborted: Error in processing.Retry sending later and monitor for successful delivery.
452Requested action not taken: Insufficient system storage.Recipient server might be overloaded. Try again later. Consider sending smaller emails or attachments.
454Requested action not taken: Message too large.Reduce email size by compressing attachments or using cloud storage links.
List of SMTP error code for soft bounces

Hard Bounce: Permanent delivery failure

Bounce CodeDescriptionAction
501Syntax error in parameters or arguments.This is often caused by incorrect configuration. Contact your administrator. 
540The recipient’s mailbox is full. Remove the address from your list or contact the recipient to clear space.This could be for various reasons, such as an inactive address. 
550The user’s mailbox is unavailable.Remove the address from your list, as it’s likely invalid.
551The recipient isn’t local to the server.The address might be misspelled or outdated. Remove it from your list.
552The action is aborted due to exceeded storage.Recipient’s mailbox is full. Remove the address from your list or contact the recipient to clear space.
553The execution of the command was halted due to an invalid mailbox name.The typo in the address is likely, removed from the list.
List of SMTP error codes for hard bounces

Email Bounces Impact Your Deliverability 

Email bounces can significantly impact your deliverability if they occur frequently. Each bounce signals to ISPs that your email practices may not be up to standard. Over time, they may classify you as a low-quality sender and deprioritize your messages. 

Maintaining a clean, updated email list and ensuring your emails are properly configured can go a long way toward preserving your sender reputation and keeping your deliverability rates high.

Proven Strategies to Boost Your Deliverability and Engagement

Deliverability is a complex beast with many components, from managing bounce rates to enhancing engagement metrics like open and click rates. To truly master your email marketing strategy, you need to understand each element’s role. 

That’s why we’ve put together “The Inbox Formula.” 

This comprehensive PDF distills the insights and strategies we’ve gleaned from operating a high-deliverability email platform

We break down the tactics that have helped us sustainably boost our clients’ deliverability and engagement rates.

Download The Inbox Formula today to start transforming your email campaigns into success stories. 

Bounce-Proof Your Email List with Campaign Refinery 

At Campaign Refinery, we’re committed to keeping your email list clean and bounce-free. 

Think of us as the master chefs of the email marketing world. Just as a chef expertly combines ingredients for the perfect dish, we mix cutting-edge technology with a meticulous strategy to ensure you get unmatched deliverability rates.

What sets us apart? 

  • Our system automatically cleans your list, removing bounce-prone addresses like invalid emails, role-based contacts, and spam traps
  • The moment an email bounces, we quarantine that address to keep your list fresh and effective.
  • We require all our clients to implement authentication protocols on their domains, so your emails will never bounce due to lack of authentication. 
  • Thanks to our excellent IP reputation, you can send millions of emails without hitting throttling limits.

What’s more, we spend time getting to know each client’s unique needs to ensure we are a good fit. 

So, if you’re interested in joining us, apply to become a customer at Campaign Refinery, today! 

Similar Posts