Guide to Email Bounce Types (2023)

A successful marketing email follows a series of steps, with each action relying on the one before it:

Delivered > Opened > Clicked > Converted

Most email marketing improvements focus on increasing conversion rates. But that won’t happen if your email never reaches your intended recipients. Understanding why an email bounces—and what to do about it—is key to ultimately driving conversions.

What is a bounce?

A bounce is an email that cannot be delivered to the recipient’s inbox. This can occur for a number of reasons, such as a recipient’s email address no longer being valid or in use, or a recipient’s mail server identifying an email as spam. 

When an email bounces, the sender’s server usually receives a message notifying it of the bounce, with details about why the email was not successfully delivered. This is known as a server mail transfer protocol (SMTP) reply. 

Email marketing software (such as Klaviyo and Mailchimp) provide reports for users (senders) on bounced emails, and, depending on the type of bounce and the software brand’s policies, they may automatically remove users from your sending list.

What is a hard bounce?

A hard bounce occurs when an email is permanently rejected and cannot be delivered, now or in the future. 

The most common reason for a hard bounce is that the recipient’s email address simply doesn’t exist. If you see this happen in your email marketing campaigns, it’s either because the customer mistyped their email address or shut down their email account. 

Sometimes, hard bounces can occur on active email addresses—for instance, if the recipient has set up a highly restrictive firewall or data policy or has completely blocked delivery to their email address. Although these usually show up as soft bounces, they can sometimes be registered as a hard bounce, depending on the restrictiveness of the recipient’s server.

Consequences of hard bounces

Most email service providers (ESPs), such as Gmail, monitor the deliverability rates of incoming emails’ domains. This is often referred to as sender reputation: Domains with a high percentage of emails delivered are considered more reputable than those that regularly send emails to invalid email addresses (which leads to hard bounces). 

This matters because email sender reputation significantly influences whether your emails are flagged as spam; maintaining high deliverability by avoiding hard bounces is essential for staying out of your recipients’ spam folder.

What is a soft bounce?

A soft bounce occurs when an email doesn’t reach its recipient for a reason that’s interpreted as temporary. This means the server has indicated that the receiving email address is real and valid but isn’t able to accept the message.

There are many reasons soft bounces occur, some within the sender’s control, some not. Here are some of the most common SMTP replies you’re likely to see due to a soft bounce, and how to tackle them: 

• SMTP Error 421. Your message was temporarily deferred by the recipient server to avoid overloading it. This typically occurs when the recipient’s server receives an unusually high volume of messages within a short time frame, usually a couple of minutes. The best thing to do in this case is to not resend the email, as it will be automatically delivered later. What you can do is review whether there are duplicates of the email address in your list. If so, it’s best to merge them—duplicate emails can overload a recipient’s server.
• SMTP Error 422. The recipient’s mailbox has exceeded its storage limit. Sending another email right away will not help here. The best thing to do is to try later, perhaps waiting till the next marketing campaign.
• SMTP Error 550. This means that the message sent is too large. To fix this, remove or edit large attachments and files from the email and try sending it again.

Although SMTP reply codes are standardized, every ESP provides its own analysis of replies and recommendations for how to handle them. If you’re unsure how to manage a specific soft bounce, consult your ESP’s documentation or speak to their customer support department.

You may also see a soft bounce if your sending domain requires DMARC authentication, an additional layer of security that requires the receiver’s email service provider to reject the email if it doesn’t meet specific criteria. If you’re sending an email from a reliable email address and receiving DMARC failure replies, it may mean that your email address has been compromised.

Consequences of soft bounces

Unlike a hard bounce, a soft bounce doesn’t immediately impact the sender’s reputation. After all, a sender can’t control if a recipient’s mailbox is full, for instance. However, repeated soft bounces from an email address will affect a sender’s reputation, and ultimately their messages’ deliverability, so it’s still important that marketers address soft bounces. 

Most ESPs have automatically enforced policies in place related to soft bounces. For example, if emails to an address soft bounce 15 times, Mailchimp considers it a hard bounce and removes the recipient’s address from your audience list. Klaviyo does the same after seven soft bounces.

Hard bounce vs. soft bounce

The main difference between a hard bounce and soft bounce lies in how senders should manage them. Hard bounces send a clear signal: Don’t bother emailing this address anymore—it won’t get through. ESPs can automatically unsubscribe email addresses that hard bounce.

Soft bounces are less clear—there are many possible reasons for a soft bounce. ESPs typically provide reports on soft bounces, including bounce messages that explain why the email was not delivered. For example, an email may not have been delivered because the recipient’s inbox was full, or because the email contained an attachment that was too large for the recipient’s server to handle.

How to reduce bounces to improve deliverability

1. Use a reputable ESP with robust bounce management
2. Implement double opt-in
3. Apply list cleaning practices
4. Avoid spammy language

Your emails can’t convert readers if they’re never delivered. And managing bounces is a key part of ensuring email deliverability. Note that you’ll never have a “perfect” list with zero bounces—bounced emails are a fact of life. What you want to aim for is a low percentage of bounces relative to your overall emails sent. Klaviyo, for example, considers more than 2% of your emails bouncing to be a problem and less than 1% a healthy benchmark.

Here are some best practices for maintaining a low email bounce rate:

1. Use a reputable ESP with robust bounce management

Reputable ESPs are more likely to be recognized by recipient servers, reducing your risk of soft bounces. Reputable ESPs typically also have automated bounce management practices, such as auto-suppressing hard bounces and applying a policy to move repeated soft bounces to suppression. Here are 11 ESPs we recommend.

2. Implement double opt-in

Double opt-in is when a user subscribes and is asked to confirm their subscription via an immediate automated email. This way, if a user misspells their email (and therefore doesn’t receive the confirmation), their mistyped email never makes the list and they get the chance to rectify the error by checking and retyping their email address into the subscription form.

3. Apply list cleaning practices

As you build your email list, it will naturally accrue email addresses that bounce back messages. This is natural: People move on to new addresses—especially if they subscribed with their business email address and then changed jobs. 

Segment subscribers who haven’t opened an email from you in 180 days and proactively remove them from your list. If they’re never going to open your emails, they aren’t valuable to you and can hurt your deliverability.

4. Avoid spammy language

Certain words can increase the likelihood of setting off spam filters and trigger a soft bounce. The most common are repeated uses of FREE (all caps), F r e e, repeated use of $$$, and “bankruptcy.”