Bounce Email Handling Essential Guide

Best Way to Handle Bounce Emails

One of the essential metrics to monitor after you send a marketing email campaign is the bounce rate. The bounce rate is the number of bounced emails divided by the total number of sent emails.

The bounce rate is directly related to the quality of your mailing list. A low bounce rate is the sign of a clean, confirmed opt-in list with active and engaged subscribers. A high bounce rate indicates problems with the list management and/or permission-based list acquisition methods.

A clean opt-in list should be generating bounce rates of between 2-3%. If your list is regularly generating higher bounce rates, it’s important that you work out why and take steps to reduce the number of bounces.

Here we’ve put together the information every marketer should know about bounce emails: why emails bounce, why it’s important to clean the list from bounce emails, how to handle bounce emails, and how to keep your bounce rate as low as possible.

So, keep reading…


1. Why Emails Bounce

Whether you are sending emails through an email service provider or through your own SMTP server, your emails are always subject to the SMTP commands for acceptance or rejection of emails by the recipient’s mail server.

Every email must successfully pass through multiple phases before it reaches the intended recipient.

At first, the emails are sent from your email sending software to your SMTP server. If you are with an email service provider, your messages are given to their SMTP server.

The SMTP server puts the email messages in the queue. Then it establishes a conversation with the recipient’s SMTP server by connecting through the port 25 to the recipient’s SMTP server.

The recipient’s mail server tells the sender’s server whether or not it is ready to accept mail. If the emails are accepted by the recipient’s mail server, they are considered as sent.

If the recipient’s server has predetermined that it is not accepting emails from the sender for whatever reason, the server rejects the message and it will subsequently bounce back to the sender.

So, when an email is returned to the sender without being accepted by the recipient’s mail server, the email is called bounce.

There are lots of different reasons why emails bounce and when it happens, a “return to sender” message is sent by the recipient’s mail server that explains the reason of the bounce.

The email marketing industry distinguishes the following bounce email types:

1. Hard bounce. It is a permanent delivery failure that occurs because the recipient’s email address is invalid or no longer in use. Typically the domain name no longer exists or it no longer has registered mail servers. But the email address can also be invalid due to typos, for example hotmal instead of hotmail.

2. Soft bounce. It is a temporary delivery failure that occurs for a variety of reasons including because the mailbox is full, recipient email server is down or offline, email is too big, email is deferred, etc. Check out our SPF soft fail guide for more details.

3. General bounce. It indicates that the mail server could not deliver the email message, but the reason is unclear.

4. Blocked. It indicates that the recipient’s mail server blocks the email from the sender’s mail server because it believes the sender is a known spammer, or the sender’s IP/domain is blacklisted, or the email message contains a suspicious attachment, or the message appears to have content that looks like spam or inappropriate links, etc.

5. Abuse/fraud feedback report. This is an email message sent to the sender by the ISPs’ feedback loops in case when a recipient clicks “This is spam/Report spam” on the message.

6. Transient bounce. It indicates that the mail server temporarily can not deliver the message, but it is still trying. Transient bounce emails do not require any action from the user. But if the failure persists, the transient bounce could be considered a hard bounce.

7. Challenge-Response message. It is an automatic response from the recipient, requesting that the sender confirms a real person is sending the message. Generally, the confirmation is completed manually by clicking on a link within the Challenge-Response message itself.

8. Auto-Reply/Out-of-Office email. It is an automatic response from the recipient.

Different types of bounce emails impact email deliverability differently. There are “harmless” bounce messages such as challenge-response or auto-reply ones. They can be ignored and deleted from the mailbox.

“Blocked” bounce emails and feedback reports are helpful in terms of understanding how the recipients treat your mailings. A lot of “blocked” emails is a sign that you should check your email program and pay attention to the message content and your sending IP/domain.

Soft and transient bounce are ok until they turn into hard bounce ones.

The most important types of bounce emails that should be addressed are hard bounce and abuse/fraud feedback reports.

They indicate that the sender uses bad list acquisition and/or list management practices and sends unwanted or irrelevant messages. This directly impacts the deliverability of future email messages the marketer will send.

2. How Bounce Rate Impacts Deliverability

In the real world, a high bounce rate leads to very unpleasant consequences that every marketer should be aware of:

1. Bad reputation. Mailbox providers monitor and watch for IP addresses that continually send messages to invalid users and they adjust the sender reputation accordingly.

2. Low Inbox placement. Mailbox providers monitor bounce rates for every campaign you send, and use that information to decide where to delivery your emails in the future.

And a lot of mailbox providers have been known to block the sender’s email domain for repeated sending to email addresses from non-existing domains.

3. Blacklisting. Frequently seen high bounce rates get the sender’s IP address land on blacklists supported by ISPs and anti-spam organizations.

4. Account suspension. Email service providers have a strict policy as to what relates to bounce and complaint rates. They will suspend the user’s account if the campaign sent by the user generates a complaint rate that is beyond the ESP’s allowed limit.

They do this because high bounce rates can land their sending IPs on blacklists, and potentially affect other users of their service.

5. Lost money. Email service providers charge you for each message you are sending. Invalid email addresses are increasing the cost of your email campaigns without any return on investment.

So, you realize that you cannot ignore bounce emails and should address them after each email campaign.

Let’s see how bounce emails are handled by through an email service provider and through a self-hosted email system.

3. Bounce Email Handling through Email Service Provider

Many marketers choose email service providers because they get a “hands off” email system where everything is pre-setup including bounce email handling.

There is nothing to configure. You just create an email campaign and send it to your subscribers. Later, you check your bounce reports.

Most providers give you a short report with the following bounce email statistics:

– Hard Bounces
– Soft Bounces
– General Bounces

Not very informative, right? It certainly doesn’t help you identify and correct any problems.

The primary reason they don’t provide a detailed report on bounces is because in many cases, you’re sharing the sending IP addresses with hundreds or even thousands of other marketers.

And the sending activity of one or more of them may cause problems with the IP address reputation or even blocking at some mailbox providers. All of this, in its turn, can lead to bounce emails.

Surely, the ESP will address the issue quickly, but you’ll never know about that problem.

But it gets worse:

You can lose VALID subscribers.

Let’s say you send out a 5-email sequence promoting an affiliate or partner’s product, and their URL happens to be on a blacklist. Well, that could cause email messages to “soft bounce.”

After a number of a soft bounces (usually 3 or 5), your email service provider will gladly remove those users from your list.

And most email service providers do not look at the “soft bounce” reason and do not reset “soft bounce” times. For example, if you had 3 soft bounces because of the full mailbox, and the 4th soft bounce occurred because of a blacklisted URL in the email, that subscriber could be removed from your list simply because the “soft bounce” threshold is hit.

Thus, as you see, there are the pros and cons of handling bounced emails through an email service provider. The pros include the ease of use and “all-in-one” system.

The cons are short bounce reports, lack of information about bounce emails and risk of losing valid subscribers because of the bounce handling process setup.

What about self-hosted email systems? Let’s see how they handle bounce emails.

4. Bounce Email Handling through In-House Email System

The primary reason why email marketers choose an in-house email solution is the ability to keep data private and build own sender reputation without depending on third parties.

In-house email systems are mostly used by advanced senders because they require configuration and knowledge about outgoing and incoming mail servers.

Once installed and configured, some in-house email systems will automatically process bounce emails for you and suppress them from your list, others will give you the list of bounces and you will have to manually exclude them from your subscriber base.

However, there are two main problems related to bounce email handling through an in-house email system:

1. Bounce emails are handled only when the computer or server where the system is installed is turned to on. That is you don’t always get a realtime bounce email processing.

2. Mailbox providers often change their bounce diagnostic codes. So, you need to wait for the software vendor to make changes to their program and then install the update.

Until that, your bounce emails are not properly tracked and you can miss a portion of your bounces.

It means that you will be trying to send to invalid addresses over and over again while you should have been removed them from your list.

It impacts your overall sender reputation and your deliverability. Invalid recipients are one of the key reasons why email reputation and Inbox placement go down.

As you see, either of the bounce handling methods (through an ESP and in-house system) is not perfect.

But here’s the kicker:

Keep reading the post to learn about the best bounce handling solution that combines ease of use and high accuracy in bounce email tracking.

5. Best Way to Handle Bounce Emails

Like we said, if you are using an email service provider, you do not need to worry about the processing of your bounce emails because they will handle it for you. But if you are managing an in-house email system and send emails in large volumes, it’s a nice idea to pass your bounce email handling to a third-party service like GlockApps.

In addition to an email spam checker, GlockApps offers a comprehensive bounce and feedback loop monitoring tool that gives you precise information about your bounces and complaints.

The GlockApps Bounce Monitor provides real-time analytics about your bounce messages. It collects bounce emails and shows you the bounce error code and the reason so that you can see why the email bounced: hard or soft bounce, block or complaint. It differentiates bounces by the sender’s email address and domain.

How to Send Email to Inbox

It also shows you a graphic diagram in order you can see spikes in your bounce rate over time.

How to Send Email to Inbox

GlockApps extracts the bounce email address and message ID of each email which generated a complaint and, where possible, the email addresses of the recipients who complained to help you detect and remove problematic and non-existent recipients.

GlockApps can also extract custom header fields from bounce and complaint emails.

This visibility allows you to:

  determine the campaigns generating the highest bounce rates;

  determine the recipients whose email addresses generate a bounce;

  predict which of your IP addresses are likely to be affected and at which ISPs;

  decide what email addresses you should keep and what email addresses you should remove from your list.

Simply put, the Bounce Monitor analytics allows you to identify the root cause of deliverability failure and answers the most vital questions such as:

Why is my content or IP blocked by Gmail, Yahoo!, or other ISPs?
What do I do with these bounce messages?
Did I end up on a blacklist?
How do I fix these problems?

Most importantly, the GlockApps Bounce Monitor makes it incredibly easy to process opt-­out for invalid users by allowing you to download a list of their email addresses. It’s essentially the magic functionality for managing your bounces.

The comprehensive and easy-to-read report is available for you every day so that you can track your bounce and complaint trends. And you can enable email notifications to get the report download links via email.

Using GlockApps Bounce Monitor is simple:

1 You forward your bounces and complaints to us.

2 We process them and give you the detailed report.

If you use a delivery service such as Amazon SES, SendGrid, Mailgun or SparkPost for sending email campaigns, you can use a direct integration of the GlockApps Bounce Monitor with the delivery service for bounce email tracking.

The GlockApps Bounce Monitor is available in the Business, Marketing, and custom subscription plans.


In the final chapter, we give the recommendations for keeping your bounce rate as low as possible that you can apply right now.

6. Seven Tips to Lower Bounce Rate

Here are seven quick tips to help you reduce your bounces and be in good standing with mailbox providers and your ESP:

1. Do not buy, rent or harvest email addresses. You can’t be sure of the quality of such lists. There is high risk of sending to a lot of invalid addresses and getting blacklisted. (And sending to people without permission is simply against the CAN-SPAM law.)

2. Use a confirmed opt-in process. Many studies show that a single opt-in process is not enough to get a quality list. To be sure that each user on your list is valid, make them confirm their subscription by verifying their email address.

3. Verify your existing list. If you have not emailed your list for a long time, it makes sense to check it for validity before sending messages to it. It may happen that a part of emails are no longer valid. Desktop software like Advanced Email Verifier or an online email service like DataValidation, BriteVerify, or FreshAddress will help you determine the majority of bad email addresses and exclude them from your list.

4. Monitor delivery by domains. If you see that only a specific ISP is blocking your emails, you can investigate the root of the problem and go through the removal process. Most ISPs list specific guidelines for resolving problems on their websites.

5. Check your message against spam filters. A lot of bounce emails happen because the message is blocked by the content filters. You can greatly reduce such bounces if you identify the problems and fix them.

You can run a delivery and spam score test using GlockApps. You’ll see your email spam score given by SpamAssassin and will be able to determine the problem elements of your email that must be addressed.

6. Remove repeated soft bounce emails. These may be abandoned mailboxes converted into spam traps. You’ll want to separate soft bounce emails from your main list and send a reactivation campaign to them.

7. Allow users to change their email addresses with you. People can change providers, and companies and thus, abandon old email addresses and get new ones. So, if you allow subscribers to update their email in your base, you’ll retain more customers and stay in good standing with the ISPs.

So, what’s the bottom line?

Just like complaints, high bounce rates spoil your sender reputation, Inbox deliverability and put your account with the email service provider at risk of being shut down.

So you need to make sure that you have a working process of removing complaining recipients and recipients whose email addresses have bounced from your subscriber list.



Julia Gulevich is an email marketing expert and customer support professional at GlockSoft LLC with more than 15 years of experience. Author of numerous blog posts, publications, and articles about email marketing and deliverability.