A bounced email is an email that was rejected by a mail server and thus is not delivered to the final recipient.
There are two types of bounced emails:
Hard bounces: permanently rejected emails
Soft bounces: temporary undeliverable emails
How to figure out why the email bounced
To understand why your emails bounced, you need to check the Delivery Status Notification message, also known as Non-Delivery Report or Non-Delivery Notification.
It’s an automatically generated report that contains information on why the delivery attempt was unsuccessful.
You can find it in your email account Inbox as a reply to your sent email.
Contents of a bounced email
A bounce message typically contains:
general information about the problem
date and time of the incident
mail server that rejected email
error code that will help you understand the technical root of the problem
link to additional information on how to troubleshoot the issue
Understanding the reasons of bounces
Below you will find the most common bounce error codes and explanations of how to manage them.
The type of the error code can be easily identified by the first digits:
4xx - Persistent transient failure (Deferrals: there has been a delay or issue with delivery. Your mail server will attempt to retry to send the message. If the retries are unsuccessful, the mail server will stop attempts).
5xx - Permanent errors (Fatal bounce back: the message was not delivered due to an error. Most likely will not be resolved by resending in the current form).
Most common 4xx errors:
421 4.3.0 Temporary System Problem. Try again later
421 4.4.5 Server busy. Try again later
452 4.2.2 User over quota; cannot receive new mail
450 Requested mail action not taken: mailbox unavailable (e.g. mailbox busy)
You may try to reach out to the recipient by other means and let them know about the issue or you can try to resend your email later.
Here you can check out how to resend an email to a bounced contact in Reply.
Most common 5xx errors:
550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient *** not found by SMTP address lookup
550 5.4.1 All recipient addresses rejected: Access denied
552 1 Requested mail action aborted, mailbox not found
554 5.4.14 Hop count exceeded - possible mail loop ATTR34
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.
These examples indicate that the recipient's address doesn't exist and it's an invalid prospect.
Sending emails to invalid prospects can hurt your sending reputation. To prevent this from happening, please use email validation tools. To ease the process of list hygiene, you can use the inbuilt email validation tool.
The other common reason for a bounce is that your message was blocked by recipient's local policy:
550 5.7.350 Remote server returned message detected as spam -> 550 permanent failure for one or more recipients.
550 5.7.1 Your access to submit messages to this email system has been rejected.
You can try to contact your recipient by other means and review your content and links before making another attempt.
Your emails can be rejected when your domain is misconfigured:
550 5.7.23 The message was rejected because of Sender Policy Framework violation.
Feel free to contact our support to check your domain or use this article to get more information about domain set up.
In some cases, your own mail server can block sending process. You can identify this when all your outgoing emails bounce.
GSuite users can see these messages:
550 5.4.5 Daily sending quota exceeded
You have reached a limit for sending mail. Your message was not sent.
Office 365 users may face this bounce:
Your message couldn't be delivered because you weren't recognized as a valid sender. The most common reason for this is that your email address is suspected of sending spam and it's no longer allowed to send email. Contact your email admin for assistance.
Most likely your provider considered your activity as suspicious and blocked the sending process.
Please contact our support team for further assistance!