Bounce Notification: What causes email bounces & how to setup bounce notifications

 In Developer

We all have received bounced emails in our mailboxes now or then. When we try to send an email to someone, we sometimes suddenly get a message stating that this mail has not been delivered. This message is called a Bounce email or Bounce Notification.

bounce notification email

Curious to know how to read those complexly written automated bounce notification emails, or what causes them, how to set up them, how they work and before all that – what are they? Read on for more info.

What is a Bounce Notification?

A Bounce Notification is an automated message and sent from an email delivery system. It is used to inform the sender that their message is not delivered to the intended recipient successfully. The notification contains a non-delivery report of a failed email, failed delivery status, an error message or what error might have occurred and the non-delivery notification. The email, that contains these messages about a failed email, is called a bounce email or bounce notification. Email bounces must be closely monitored and managed.

Reasons for a bounce notification

Reasons for getting an email bounce notification can be many; from email address being changed, a domain has changed, the recipient's mailbox is full, etc. We can categorize the email bounces into two main categories:

  • Hard Bounces – A hard bounce of an email is caused by permanent non-deliverable conditions such as the recipient's email address is missing or invalid. Other reasons for a hard bounce to occur are the non-existence of the recipient’s domain, unknown recipient, error in typing the recipient’s email address, blocking of your email server by the recipient’s email server, or any kind of network glitch at the recipient’s end.
  • Soft Bounces – If an email message is able to find the recipient’s email server and recognize the address, but it is not delivered to the recipient’s inbox and bounced back before delivery, it is classified as a "Soft Bounce." The reasons can include: the recipient’s inbox is full, the recipient's email server is down for some reason, or the email box is abandoned by the user. After a few attempts at delivery and soft bounces, it will likely be transformed into a hard bounce.

hard bounce soft bounce

Some of the situations that can trigger bounce notifications, upon failure of email delivery:

  1. Unknown user: The email address no longer exists, the email provider is shut down or the email address is not valid (or a portion of the email address).
  2. Full Mailbox: Each email account has a storage limit and if the limit is exceeded, the recipient's server will reject any incoming email. resulting in a bounce notification in the sender's email box.
  3. Too Large Message: The size of an email account is defined by your service provider as well as the recipient's service provider. If you have tried to send a mail that exceeds the limit, which includes the header, text message, attachments, etc., the recipient's server will reject the incoming mail for being too large to accept.
  4. Did not Find Domain: This means that the domain the email address used does not exist or has been shut down.

These are the main reasons a user receives an email bounce notification. Other causes include: email is trapped in a routing loop, connection timed out or network error, domain internal issues like switching hosts, blocking of certain IP addresses from recipient's mail server, and other unknown or undefined reasons.

How does the bounce notification work?

To understand the functionality of bounce notification, we need to understand the email flow first.

In the above flowchart, we can see the journey of the email along with the components. After passing through all the components, there is a handshaking stage. The SMTP server of the intended recipient takes a big role here, as it recognizes the sender's domain and makes a contact with the receiving server. The recipient server goes through the message and checks the parameters mentioned above to give a green signal to the mail. If any of the parameters is missing, wrong or unknown, the recipient's server rejects the email. Now the mail is sent back to the sender's server and the bounce notification is sent to the sender's inbox.

But how do you know if your email was actually delivered? There's a lot to learn.

How to read the bounce notification emails?

Bounce messages have specific formats that are predefined by the email server. A bounce notification can be a MIME (multipart report message). It has three parts:

  1. An explanation in a human-readable format.
  2. The delivery status which can be parsed by a machine. This includes a “name: type, value” parameter.
  3. The original notification message indicating the possible causes.


How to read bounce notification

Image Source: Wikipedia

How to set up bounce notifications

  • To set up a bounce notification in Pepipost, navigate to Settings -> Customize -> Bounce Notification -> View Settings.
  • You need to create a Custom Envelop. Go to Domain Management -> Settings and create a custom envelop to get bounce notifications.
  • The bounce forwarding within your account is enabled. Now each time, when a bounce appears, the system will automatically notify you. It will forward a bounce notification on the email address, which you have set for bounce forwarding.

To handle these notifications, you need to configure your app to receive bounces. For that, Real-time Event Notification API has to be enabled in the application. Also, the list of email bounces can be edited and retrieved through Web APIs of your service provider. Hard and Soft Bounces can be handled differently, both handled through your account or APIs provided by your service provider.

The Benefits

One of the additional benefits of setting up these notifications is that it will notify you about clicks, opens or anything that happened to the emails. You can easily calculate the response rate and improve your mailing campaign’s strategy accordingly. Being able to quickly see bounce notifications coming in may also alert you to issues in your data.

Related Resources



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