Emails have been an obligatory part of our daily life. Almost every computer users will be using emails in their day to day life. There will be at least one situation that we have got a bounce back email in our email communications. What is really a bounce back message and what are its causes? It is very important that one should be aware of bounce back emails as it will help to avoid many confusions in dealing with the emails.

What is a Bounce Back Email?

A bounce back email is an email which will be coming to our email inboxes after the failure in delivery of a particular sent email. However, every delivery failure will not cause a bounce back. The bounce back message is generated either by our email server or the email server of the destination account. It is a method to let the email sender know that something has happened to interrupt the email delivery. The email bounce back is a standard method to provide an error. This will help in diagnosing the email issue and understanding the actual reason of interruption in delivery of a particular email.

Why Do Emails Bounce Back?

To see why a bounce back occurs, let us go through the step by step process  in email communication. These are the processes carried out during email communication.

1) The email is created by filing at least from and to addresses fields.

2) The email is sent by clicking the ‘send’ button.

3) The email is given to the mail server for delivery.

4) The email is attempts to deliver from the sender’s email server to the destination email server.

5) The email is delivered successfully by the receiver’s email server to their inbox.

Mail servers uses MTAs (Mail Transfer Agents) to handle email delivery. If there is a temporary error like incorrect email address, the remote MTA will send a bounce back email addressing the particular problem to the sender. This will contain a specific error message and its error code.

Reasons for Bounce Back Emails

There may be many reasons for bounce back emails. We are going to explain six important reasons for  bounce back email.

1) A non-existent email address

2) Undeliverable email

3) Mail box full

4) Auto reply

5) Blocked email

6) Malicious contents

A Non-Existent Email Address

If the bounce email is marked as “non-existent email address”, It means that the email address we have typed in to the to address field is not a valid one. It can also be a typo or an existing user has left and is not available at the moment.

Undeliverable email

If the bounce emails are marked as “Undeliverable”, then it means that the email server of the receiver is temporarily unavailable. The email server may not be found because of the server is overloaded or could be under maintenance.

Mail box full

Suppose the receiver’s inbox is full and there is no space for new incoming emails, the sent emails will not be delivered and a bounce back email will be generated.

Auto reply

Unlike other categories, this type of bounce back emails mean that your email has been delivered successfully. Sometimes, people will enable the auto reply option for their email accounts and every emails to those accounts will be bounced.

Blocked email address

There is a possibility that the receiving email account owner has placed the sending email address into a blocked list. If an email account is in the blocked list, then the emails from the particular sender will not be delivered into the receiver’s inbox, so that a bounce back will be generated.

Malicious contents

Sometimes, emails will not be delivered due to malicious contest. There are some specific types of files which we cannot send with an email. If we try to send some executable files like files with a .exe or .wsf extension, as attachments with the emails, they will not be delivered, but will bounce back with a message like email virus detected, etc.

 Types of bounce back emails

There are commonly two types of email bounces, the soft bounce and the hard bounce. Let us further explain each type now.

Soft Bounce

There are many reasons for the soft bounce. Suppose the receiver’s email inbox quota has been exceeded. If we try to send an email to that account, it will cause a soft bounce back. The soft bounce can occur also when the remote mail server is temporarily unavailable. In these cases, the sender’s mail server will be trying to resend the email to the remote mail server. A soft bounce is a temporary problem and it is caused by the receiver’s email server. It will alert the sender about the issue so that the email can be sent at a later date.

Hard Bounce

Hard bounce can be occurred at the following conditions. If the ‘to’ address entered is incorrect, so that the email could not be delivered. It will cause a permanent error and in this situation, a bounce back email will be coming. Sometimes a hard bounce cannot be a permanent error. In such situations, it is essential for the email server to inform the sender that the emailed user doesn’t exist on the server and it will not be delivered at a later time.

Blacklisted Bounce Back Messages

In some situations, email servers can be flagged as spam. In such cases, email servers can be blacklisted. This flagging may not be true always, but if the email is being blocked because of the server is blacklisted, then we need to send a delist so that the server is delisted. Email servers tend to be blacklisted when they are relaying spam.

Why Does a Server Get Blacklisted?

It is not rare that a mail server is being blacklisted. This can be happened especially in shared servers. The primary reason of an email server is being black listed is spamming. Sometimes, email forwarders will cause the servers to end up in a black lists. Suppose [email protected] is forwarding to [email protected] and [email protected] is spammed, our mail server could forward that onto example1. In such cases, the example1’s server might see our email IP is relaying spam, even though it is not originating the spam. In such cases, our email server is being blacklisted. In the case of server black lists, it should be delisted.

These are the common causes of the email bouncing. Email bouncing can occur due to several reasons as we have mentioned earlier.