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 check your bounce reasons in several places:
Your mailbox (review the bounce notifications directly).
Reports page → hover over the Bounced metric to see the detailed breakdown.
People tab → export the full list of bounce reasons.
Bounce email structure
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
We’ve grouped the bounces into several conditional categories. Below, you’ll find a list of these categories along with some examples.
Category | Example | Explanation | Action |
Undeliverable | 550 5.1.1. User unknown, Mailbox not found, No such user, Recipient address rejected, Invalid recipient | Invalid email / Mailbox disabled. The recipient’s email address does not exist or the mailbox has been deactivated and cannot receive messages.
| Validate your prospect list prior sending. |
Mailbox full | 552 5.2.2 Mailbox size limit exceeded, User has exceeded quota, Disk full, Recipient's mailbox is full, | The recipient’s mailbox has reached its storage limit and cannot accept new messages until space is freed up.
| Try later or remove prospects from your contact list. |
Authentication failure | 550 5.7.1 DMARC policy, DMARC check failed, rejected by DMARC, missing SPF record, no SPF record found | The receiving server could not verify the sender’s identity because one or more email authentication checks failed, so the message was rejected.
| Check your email authentication (SPF, DKIM, DMARC) |
Spam rejection | 550 / 554 Spam detected, Blocked by spam filter, Message flagged as spam, High probability of spam | The receiving server identified the message as suspicious or spam based on its filtering rules and refused to accept it. | Try changing your content, remove tracking or other links / images and try later |
Policy violation | 554 / 550 5.7.1 Blocked by policy, Recipient policy, Administrative prohibition, Message rejected due to local policy | The message was rejected because it violated the recipient server’s local or administrative policies (e.g., content restrictions, security rules, or recipient-specific settings). | Contact this prospects by other means |
Temporary failure | 421 / 451 Connection timed out, No answer from host, Server busy | The message could not be delivered due to a temporary issue on the recipient’s server (e.g., timeout, no response, server overload, or temporary system error) and may be retried later. | Try re-sending email later |
Sender's block: Gsuite | 550 5.4.5 Daily sending quota exceeded You have reached a limit for sending mail. Your message was not sent. | Can occur when you suddenly increased your sending volume or received a lot of spam complaints from Gsuite recipients | Pause sending for the next 24-48 h, start with low volumes and increase slowly |
Sender's block: Office | Your message couldn't be delivered because you weren't recognized as a valid sender....
'550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP. For more information, please go to http://go.microsoft.com/fwlink/?LinkId=526653 AS(7230)
Trial tenants: 550 5.7.232 - Your message can't be sent because your trial tenant has exceeded its daily limit for sending email to external recipients (tenant external recipient rate limit) Non-trial tenants: 550 5.7.233 - Your message can't be sent because your tenant exceeded its daily limit for sending email to external recipients (tenant external recipient rate limit) | Your provider considered your activity as suspicious and blocked the sending process.
Office has introduced new sending limits per tenant | If you receive "..you were not recognized as a valid sender", navigate to How to Restore Your Office 365 Email Account After Suspension for a guideline
If you receive "...Traffic not accepted from this IP...", pause your sending process for 1-2 days or contact MS support and share this bounce.
In this case, you will still be able to send manual emails from your mailbox — the block applies only to third-party email automation.
Please, refer to Introducing Exchange Online Tenant Outbound Email Limits article for details |
Other |
|
| The bounces were not categorized into any category. |
You can also find the specific bounce category at People tab by using a Smart filter:
For provider's block you can also use this filter:
If you have any further questions, please feel free to contact our support team — we’ll be happy to assist you.