Hard Bounce & Soft Bounce: A Complete Guide to Email Marketing Bounce Rates

The Email Marketing Funnel 

When you create a email marketing The first performance indicator to be optimized, even before the Open Rate, is the rate of deliveryor deliverability, the ability of your message to reach its destination.

If you think about it, from the time it's sent to the time your recipient reads it, your email goes through a pathway shaped like funnelwhere, not surprisingly, the entry point is deliverability:

An important metric in email marketing is the Bounce Rate: as you know, since it's a metric that, with due differences, we also find in Google Analytics (by the way, do you know the modified rebound rate), a bounce indicates a bounce, in our case the email sent was not delivered and the email server sends it back to the sender.

 Bounce rate is an important metric to monitor and optimize in email marketing because as we will see:

  • has a significant impact on the reputation of the sender and the resulting deliverability rates;
  • can lead to unpleasant account lockouts by theEmail Service Provider.

How a Bounce Happens: The Role of ISPs and ESPs 

As we have seen, when an email cannot reach an email server, it is called a Bounce.

Different Internet Service Providers (ISPs) can bounce emails based on their own rating systems.

When this happens, the server gives a reason to the ESP who in turn draws the consequences on how to treat that email address.

As we'll see in a moment, there are two different types of bounce, soft and hard bounce:

  • Soft Bounce: A Soft Bounce identifies a temporary deliverability problem. In this case, unlike a hard bounce, the email service provider identifies the email address in the report of the sent campaign as a soft bounce;
  • Hard Bounce: A Hard Bounce indicates a reason why an email cannot be delivered permanently. In most cases, email addresses marked in Hard Bounce are placed on a "suppression list" by your ESP and automatically and immediately excluded from all future mailings. 

How ESP Identifies Soft Bounce and Hard Bounce

Certain email marketing software may categorize an email address that experiences numerous soft bounces in different campaigns or during the sending of the same campaign as a hard bounce.

Usually, if a Soft Bounce occurs, they will still try to reach the recipient's server multiple times over a period of time: after making Soft Bounce a number of times your email marketing software may decide to classify the email address as Hard Bounce.

An agreement signed in the early 2000s between ESP and ISP (Internet Service Provider) states that there must be 3 consecutive bounces and 15 days between the first and last bounce to determine whether an address can be considered unavailable.

There may also be other factors that determine the classification in Hard Bounce such as:

  •  the engagement level of the contact; 
  • the fact that you wrote yourself an optin through the software.

Contrary to what you might think, a bounce can occur either while sending at different points (https://wordtothewise.com/2014/05/smtp-level-rejections/) as well as subsequently, resulting in what is called a Asynchronous BounceIn the latter case, it is even more difficult for the software to understand the reasons for the bounce. 

Bounce Readers and SMTP

Occasionally there may be cases where an email is incorrectly identified as Hard Bounce: in these cases it is important to learn to read the Bounce Readers, or SMTP responses to understand the causes of what created a hard bounce.

  • For example, if an SMTP response includes terms like denied blockedThis could mean that the email has been blocked due to spam filters.
  • Sometimes an RFC code is used to indicate the reason for the bounce in this case Hard Bounces are identified as 5xx and Soft Bounces as 4xx.
  • When the email does not exist you usually get a 550, 551 server error.

3 Macro Reasons for a Soft Bounce

 Here are some typical reasons why an email address may incur a soft bounce:

  • SOFT USER: Some reasons may depend on the recipient's mail box: for example it may be full, not configured correctly or inactive. Some Mail Boxes in fact have limitations in terms of capacity and once the limit has been exceeded leads to a Bounce of incoming mails. Or it may be too big for your recipient's Inbox, for example due to the presence of heavy images or GIFs.
  • SOFT TECHNICAL: in some cases the problem depends on the server that could be in TIMEOUT, down or off-line,
  • SOFT BLOCK: In other cases the Soft Bounce may be because the email message may have been blocked due to the content of the server's Policy. It could be that the recipient's email provider has blocked you because many users have identified your emails as spam or you are blacklisted. 

3 reasons for Hard Bounce

A hard bounce indicates a reason why an email cannot be delivered permanently. 

In most cases, email addresses marked in Hard Bounce are automatically and immediately deleted from your audience and excluded from all future campaign mailings.

But why does an email end up in Hard Bounce?

The reasons are basically as follows:

  • no longer exists: for example, the syntax of the emails is not correct.
  • the server's blocking reception, so let's see why.


The server may have blocked the reception of your email as it is sent from free services such as Gmail or Yahoo. In fact, to reduce the phenomenon of spam many email service providers use the DMARC Policy. https://dmarc.org/ (or Spf Dkim) or email authentication policies that if they are not exceeded can lead to a bounce.

From this point of view it would be appropriate that the sender entered is the email related to your domain, for example: [email protected] (don't write to this email, it doesn't exist! 😉 )

What is a good Good Rate? When is it high?

Before we go into how to improve your Bounce Rate and avoid running into Soft and Hard Bounces let's understand:

  • when it comes to high bounce rate;
  • the risks of a high bounce rate. 

Usually in the literature on the subject We consider an optimal bounce rate in email marketing below 1% and vice versa a high bounce rate above 2-3%.

IP Reputation and Sender, Deliverability

As we anticipated not taking care of the bounce rate aspect in your email marketing campaigns can basically lead to a negative impact in the reputation of the sender (negatively affect the reputation of your IP) and consequently in the deliverability rates of your emails with a possible suspension of the account by your email marketing software.

Account Suspension

A Bounce Rate above certain thresholds could lead to an account lockout: in this case the suspension tends to be temporary. The software tries to understand if your list has been acquired in a legitimate way and recommends some list hygiene operations, such as cleaning of the list in order to clean it up.

A high volume of addresses returning a bounce may in fact be an indicator of non-privacy compliant or even illegal contact acquisition.

Bounce Rate in email marketing: 2 case studies

2 cases from this point of view on 2 different software.

Case 1: Problem and Solution

A client sends out a couple of newsletters from not-so-clean lists: this phenomenon has led to a very high bounce rate, in our case around 30% 40% which has led to SendinBlue to block the account temporarily, asking where the list came from and recommending that it be "cleaned up".

Hard Bounce Soft Bounce Sendinblue

Solution: Acquire clean, quality contacts

 Understand then that the first step to avoid incurring other degrees of Bounce is just to keep your email lists clean.

From this point of view I can only advise you to avoid buying lists, but acquire contacts, e.g. through initiatives of Lead Generationthat they guarantee you:

  • to acquire real e-mail addresses;
  • to own your list;
  • to optimize costs by no longer being dependent on the lists of others.

Another aspect that can help you keep a clean list is to use Double Opt In on sign up although personally you may have a large amount of people not confirming the email address thus losing a large amount of leads.

Some email marketing software, such as MailPoet e Drip use double optin by default to optimize delivery rates.

An alternative is when filling out the email in the form to force you to double-check the email to avoid typographical errors.

Case 2: Problem and Solution

A client sends a newsletter to a list that he hasn't used for a very long time and this has led to a high degree of Bounce and the consequent temporary blocking of the account of Mailchimp.

Solution: if you haven't heard from your contacts in a long time, consider a list cleanup, re engagement.

From this point of view 2 tips:

  • clean the list before sending it with software like MailFlossintegrated in many email marketing software;
  • think about initiatives to involve the inactive leads.
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