Question
I failed to send an email and received a Non-Delivery Receipt (NDR) error email. Can I determine the cause of the delivery failure from the content of the error email?
Answer
Yes. By checking the content of the error email, you may be able to identify the cause of the delivery failure.
* If the email was rejected by Email DLP, please refer to this article.
You can also provide the error email to HENNGE One Technical Support for investigation.
However, as it may take some time to receive a response, please check if your case matches any of the common types of error emails listed below.
An error email (NDR) is an automatically returned message from the mail server
that is sent when an email could not be delivered to the recipient for some reason.
The message usually contains the reason for the failure, so you can check for keywords
and determine whether your case matches.
In addition, error emails sent from major email services such as Gmail often include a URL with more details, so please also refer to the linked page.
Main formats of error emails
<Recipient address>: host <Recipient mail server> said:
<SMTP code> <Message *The error reason is included here> (in reply to <Response timing>)
User Unknown <User Unknown, Unknownuser, NoSuchUser, etc.>
This message is returned when the local part of the recipient's email address (the part before the @) does not exist.
The main causes are an incorrect email address or the email address no longer exists, so please check the recipient's email address and try resending.
Note that mobile carriers tend to have stricter spam checks and may return a response such as "Unknown user" even if the actual error is different (for example, even if the recipient address exists).
If you receive this error even though the recipient address is correct, there is nothing you can do on the sender side. Please contact the recipient and ask them to allow your email.
Example:
<xxxx.xxxx@xxxx.co.jp>: host aspmx.l.google.com[000.000.000.000] said: 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 https://support.google.com/mail/?p=NoSuchUser y27-v6si3456678mkes.982 - gsmtp (in reply to RCPT TO command)
<xxxx.xxxx@xxxx.co.jp>: host <Recipient server FQDN[IP address]> said: 550 5.7.1
<xxxx.xxxx@xxxx.co.jp>: Recipient address rejected: User unknown (in reply to RCPT TO command)
Diagnostic information for admins:
Generating server: xxxxxxxx.prod.outlook.com
xxxx.xxxx@xxxx.co.jp
Remote Server returned '550 5.1.1 RESOLVER.ADR.RecipNotFound; not found'
Host Unknown <Host or domain name not found, Host not found, etc.>
This message is returned when the host name (the part after the @) of the recipient's email address does not exist.
The main causes are a typo in the recipient's email address, a non-existent domain, or a DNS server error. Please check the recipient's email address and try resending.
If you receive this error even though the recipient address is correct, there may be an issue with the recipient domain. Please contact the admin of the recipient server.
Example:
<xxxx.xxxx@xxxx.co.jp>: Host or domain name not found. Name service error for name=xxxx.co.jp type=A: Host not found
Diagnostic information for admins:
Generating server: xxxx.jp
xxxx.xxxx@xxxx.co.jp
Remote Server returned '< #5.4.4 X-Postfix; Host or domain name not found.
Name service error for name=xxxx.co.jpp type=A: Host not found>'
Email Size Exceeded
This message is returned when the size of the sent email (including attachments) exceeds the size limit set by the recipient server.
Please upload the file to a storage service such as Secure Transfer or Google Drive, share the file, and resend the email with a smaller size.
Example:
The error that the other server returned was: 552 552 message size exceeds maximum size (state 18).
<xxxx.xxxx@xxxx.co.jp>: message size 14840231 exceeds size limit 10240000 of server <Recipient server FQDN[IP address]>
552 sorry, that message size exceeds my databytes limit
Mailbox Full
This message is returned when the recipient user's mailbox is full.
The main reason is that the recipient's mailbox (mail spool) has exceeded its capacity and cannot receive new emails.
Please ask the recipient user to delete emails from their server mailbox and then resend your email.
Example:
<xxxx.xxxx@xxxx.co.jp>: host <Recipient server FQDN[IP address]> said: 552 5.3.0 Message would exceed remaining quota (in reply to RCPT TO command)
<xxxx.xxxx@xxxx.co.jp>: host alt1.aspmx.l.google.com[000.000.000.000] said: 452-4.2.2 The email account that you tried to reach is over quota.
Please direct 452-4.2.2 the recipient to 452 4.2.2 https://support.google.com/mail/?p=OverQuotaTemp p23si29386027hen.76 -gsmtp (in reply to RCPT TO command)
<xxxx.xxxx@xxxx.co.jp>: user is over quota
Rejected (for Google Workspace)
When the recipient is using Google Workspace, for security reasons, executable files that may contain malicious code cannot be sent or received.
From the Google Help page, please check the following items at https://support.google.com/mail/?p=BlockedMessage:
・File types that cannot be sent or received
・ZIP files within ZIP files (double ZIP)
・Maximum file size
Example:
<xxxx.xxxx@xxxx.co.jp>: host aspmx.l.google.com[000.000.000.000] said: 552-5.7.0
This message was blocked because its content presents a potential 552-5.7.0 security issue.
Please visit 552-5.7.0 https://support.google.com/mail/?p=BlockedMessage to review our 552 5.7.0 message content and attachment content guidelines. p23si29386027hen.76 -gsmtp (in reply to end of DATA command)
Rejected
The recipient's mailbox is configured to reject emails sent from external sources, or the recipient's settings are blocking your email.
Please contact the admin of the recipient server.
Example:
<xxxx.xxxx@xxxx.co.jp>: host xxxxx.mail.protection.outlook.com[000.000.000.000] said: 550
5.4.1 Recipient address rejected: Access denied xx(xxxxxxxxx) [xxxxx.prod.protection.outlook.com] (in reply to RCPT TO command)
Recipient Server Timeout
This message is returned when the connection to the recipient server is disconnected.
Possible causes include a temporary issue with the recipient server or a policy on the recipient server that prevents email delivery.
Please contact the admin of the recipient server.
Example:
<xxxx.xxxx@xxxx.co.jp>: delivery temporarily suspended: connect to <Recipient server FQDN[IP address]>:25: Connection timed out
Rejected by Microsoft Email Sending Policy
When sending more than 5,000 emails per day to Microsoft's consumer webmail services such as Outlook.com (domain names: @outlook.com / @outlook.jp / @hotmail.com), emails that do not comply with Microsoft's guidelines will be rejected.
Please refer to the following Help Center article for more information.
Guidance on responding to Microsoft's new email sending policy
For further details, please contact Microsoft.
Example:
<xxxx.xxxx@xxxx.jp>: host.xxxxx.xxxxx.xxxxx[000.000.000.000] said: 550; 5.7.15 Access denied, sending domain [SendingDomain] does meet the required authentication level.