An email bounce occurs when a message is rejected by the receiving mail server. The specific reason for the email being returned can sometimes be confusing for end users. Therefore, it's worthwhile to delve deeper into the concept of email bounces.
It is useful to receive a notification when an email fails to reach the recipient. This allows you to take action, such as reaching out to the recipient through another method or correcting any typos if necessary.
Email bounces can easily be identified by the sender (mailer-daemon@) and the subject line (terms such as Non-Delivery Report/Receipt, Delivery Status Notification, or Non-Delivery Notification).
Email bounces can generally be categorized into two types: Soft Bounce and Hard Bounce.
Soft Bounce
This type of bounce is usually temporary. It can occur, for example, when the recipient's mailbox is full, there is a temporary network issue with the receiving mail server, or when the message is too large.
Hard Bounce
In contrast to a Soft Bounce, a Hard Bounce is permanent. This type of bounce typically occurs when the recipient's email address or domain no longer exists (watch out for typos), or when the sending mail server is blacklisted.
An email bounce is always accompanied by a three-digit (status) code and a specific subject, such as "550 No such recipient here." Sometimes the code is extended with an additional code, such as "550 5.1.1 The email account that you tried to reach does not exist."
These codes are standardized according to RFC 3463. By consulting this RFC or using a search engine, you can often quickly find out why an email message didn’t reach the recipient.
Example:
In the example below, the bounce returns the code '550 5.1.1 The email account that you tried to reach does not exist'.
The external server's response was:550 5.1.1 The email account that you tried to reach does not exist. Please try double-checking the recipient's email address for typos or unnecessary spaces. Learn more at Google Support p52si23555697wrc.192 - gsmtp
Final-Recipient: rfc822; dave-doesnt-exist@ digiter.com Action: failed Status: 5.1.1 Remote-MTA: dns; aspmx.l.google.com. (2a00:1450:400c:c06::1a, the server for the domain digiter.nl.) Diagnostic-Code: smtp; 550-5.1.1 The email account that you tried to reach does not exist. Please try 550-5.1.1 double-checking the recipient's email address for typos or 550-5.1.1 unnecessary spaces. Learn more at 550 5.1.1 Google Support p52si23555697wrc.192 - gsmtp Last-Attempt-Date: Mon, 13 Mar 2017 01:05:50 -0700 (PDT)
The subject of the bounce itself is fairly clear. According to the RFC, it is clarified as follows: 5.XXX.XXX Permanent Failure. A permanent failure is an error that will likely not be resolved by simply resending the message in its current form. A change must be made to the message or destination for successful delivery.
The additional code provides specific information: X.1.1 Bad destination mailbox address. The specified mailbox in the address does not exist. For Internet email addresses, this means that the part of the address to the left of the "@" symbol is invalid. This code is only useful for permanent errors.
From this bounce, you can infer that the recipient's email address is incorrect (Hard Bounce).
It is possible to receive an email bounce notification for an email message that you did not send. This phenomenon is known as email spoofing, where spammers send emails with a forged sender (your email address) to a recipient who is unreachable.
