Understanding Bouncing Emails: A Short Guide

What is a bouncing email?

Bounced emails are emails that did not reach the intended recipient for some reason. Within this broad explanation we can see two different kinds of bounces: soft bounces & hard bounces.

Soft Bounces

Soft bounces occur when the reason for the email to bounce is a temporary issue. This can happen when the recipient’s mailbox is full, or the email is too large. It can also occur when their email server is temporarily down.

Hard Bounces

Hard bounces will happen when the email address you are trying to reach does not exist or is invalid. They can also happen when the email contains blocked attachments, or your domain/email address has been blacklisted.

How can I tell which emails are bouncing?

Whenever you use an Email Service Provider (ESP) to send emails they will normally do two things.

First, they will isolate and remove and emails that they know to be bounces. This is very useful as it will prevent you sending emails which will not be delivered. Sending emails to addresses that are known to bounce will ultimately hurt your sender reputation.

Secondly, after you have sent an email blast, the software will let you see and download a list of emails that it identified as bouncing. Most ESPs can tell you if this was due to soft bounces or hard bounces.

How do we measure bounces?

Your ESP will refer to a ‘bounce rate’. This is simply the ratio of bounced emails compared to the size of your list.

Why is the bounce rate important?

A bounce rate of 5% or less is considered acceptable in most cases, whereas anything above that would cause most ESPs to take action. We do occasionally see that some of them allow a bounce rate up to 8%. If you have too high a bounce rate your ESP could change you to a server with a worse reputation (which will impact your deliverability rates). Or they could refuse to send more emails if your bounce rate was very high. For example MailChimp has a reputation for doing this. To find out more details we recommend that you talk to your ESP directly. They will inform you about their individual guidelines.

Bounce rate and your Sender Score

This score is an indication of how trustworthy emails sent from your domain are. It can vary between 0 and 100. If your score is higher, more people will receive your message, but if the score is lower, then your email will be blocked, or sent to spam by most recipient mailboxes. This in turn will cause your sender score to drop even further. Ultimately, it will make it even trickier for you to reach your prospects.

Bounce rate has a direct impact on your sender score because it (among other factors) reduces your sender score if it is too high.

Deal with bounces immediately

The best practice is to remove any hard bounces from your database after one occurrence. You may feel reluctant to do this, but you need to understand that any emails you try and send the contact won’t be received by them. As part of a wider cleaning programme you may choose to clean your data (perhaps there are some obvious errors, misspelt name in the email etc.).

Soft bounces require a different procedure. We recommend trying to reach them 3 times before accepting that there is a bigger problem with the contact. At this point you can treat them as a hard bounce.

These actions will reduce the size of your mailing list, but you will be left with a cleaner, healthier more relevant list.

Avoiding a high bounce rate in the first place

The gold standard for email lists is to only send to a list that you have built yourself using the double opt-in method. This is where a person enters their email address, and then receives an email with a link that they have to click on to confirm that they want to receive emails from your company.

In the perfect world companies would invest time and money in cleaning their data in an ongoing rigorous process. Churn (especially in the Telco and Media segments) is high, and it takes a lot of resources to keep your data clean.

What about sending to purchased lists…?

We recognise that not every marketing campaign has a carefully collected and curated database at its heart.

Business development lists can be created and purchased. We always advise our clients to be extremely cautious about purchasing lists. Choose a reputable agency, agree exact targets, don’t use a ‘low cost’ company (what you pay is really what you get) and ask for quality assurances.

Before you send your email campaign we recommend that you use an email verification software to remove the invalid emails. This will go a long way in reducing your bounce rate.


Bounce rate is an important factor to consider. A bad bounce rating will impact your marketing campaigns and sender reputation. You must practise good list (and database) hygiene both before and after sending emails. If your database is of doubtful quality, you need to take steps to correct it or choose a company who can help you.

We’d like to invite our clients to contact us if you need further advice on how to improve your email deliverability.

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