This guide helps you identify and correct most email errors encountered after sending an email to a recipient. Refer to this other guide if you are looking for information about errors encountered when setting up an email address in a software/email client.

Introduction

• Generally, an error message related to email occurs due to…
  • … an incorrect configuration of the domain name used with your email,
  • … a problem with your recipient's (email provider).
• In all cases, a delivery failure report (Mailer Daemon or Mail Delivery Subsystem) containing the explanation is sent to the sender.

Find and identify the email error

When your message fails to reach the recipient, an initial failure report (Temporary Delivery Failure Report) is sent, indicating that the server will attempt to resend your message several times.

A final non-delivery report (Final Delivery Failure Report or Undeliverable Mail) issued by the recipient's server is sent to the sender (via the Infomaniak mail server, which is why Infomaniak is mentioned at the beginning of most error messages you may receive) to inform them that the email could not be delivered even after several attempts.

The notification always contains the reason for the failure. Example below:

This is the mail system at host smtp-1-1234.mail.infomaniak.ch.
I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can delete your own text from the attached returned message.
The mail system anna.a@abc.xyz: host mxbw-abc-xyz.abc-ab12.abc.ch[123.456.789.00] said: 550 5.1.1
anna.a@abc.xyz recipient rejected, account administratively disabled (in reply to RCPT TO command) 

Therefore, here, the address of your correspondent is visibly suspended by their email provider (account administratively disabled).

Sometimes, you need to look for the reason behind the error message you received within its attachments (often a plain text file). Example:

Reporting-MTA: dns; mxbw.abc.ch‍ [123.456.789.00]
Received-From-MTA: dns; smtp-1-1234.mail.infomaniak.ch [12.345.678.99]
Arrival-Date: Tue, 01 Aug 2023 15:49:22 +0200
Final-recipient: rfc822; anna.a@abc.xyz
Diagnostic-Code: smtp; 552 RCPT TO:anna.a@abc.xyz‍ Mailbox disk quota exceeded
Last-attempt-Date: Tue, 01 Aug 2023 15:49:22 +0200

Here, the mailbox of your external contact at Infomaniak appears to be full (Mailbox disk quota exceeded).

Most common cases

SPF Failure

The SPF (Sender Policy Framework) record is not configured correctly:

Subject: Delivery Status Notification (Failure)
From: Mail Delivery System mailer-daemon@domain.xyz
To: Anna.A anna.a@abc.xyz
This is an automatically generated Delivery Status Notification.
Delivery to the following recipient failed due to SPF failure:
anna.a@abc.xyz
Technical details of SPF failure:
The sender's domain domain.xyz does not have a valid SPF record. Please contact your email administrator to correct the SPF configuration.

Solution: check the configuration of your domain name, and in particular the SPF record if your website is hosted on Wix or another provider.

Unknown User

The email address specified in the message is invalid or does not exist:

Subject: Delivery Status Notification (Failure)
From: Mail Delivery System mailer-daemon@domain.xyz
To: Anna.A anna.a@abc.xyz
This is an automatically generated Delivery Status Notification.
Delivery to the following recipient failed permanently:
anna.a@abc.xyz
Technical details of permanent failure:
The email address does not exist. Please check the recipient's email address and try again.

Solution: carefully check the recipient's email address and correct it if necessary; ensure that you are using a valid email address for the recipient.

Temporary Failure

The recipient's mail server is experiencing temporary issues or is unavailable.

Solution: Wait a while, then try sending the message again later. If the problem persists, contact the recipient's technical support for assistance.

Blocked by SPAM Filter (or Content Rejected)

The recipient's mail server blocked the message due to anti-spam filtering rules:

Subject: Delivery Status Notification (Failure)
From: Mail Delivery System mailer-daemon@domain.xyz
To: Anna.A anna.a@domain.xyz
This is an automatically generated Delivery Status Notification.
Delivery to the following recipient failed due to the message being rejected as spam:
anna.a@domain.xyz
Technical details of delivery failure:
The content of the message triggered the spam filters of the recipient's email server. Please review the message content and try again.

Solution: check if your message contains elements that may be considered spam (keywords, suspicious links, etc.). Contact the recipient to verify if the message was filtered into the spam folder.

Refer to this other guide about outgoing spam and this other guide about incoming spam.

Bad Reputation

The recipient's mail server blocked the message due to anti-spam filtering rules:

gmail-smtp-in.l.google.com said: 550-5.7.1
Our system has detected that this message is likely suspicious due to the very low reputation of the sending domain.
To best protect our users from spam, the message has been blocked.
Please visit {{URL_5}} for more information.

Solution: check the reputation of the domain and determine if one of your email addresses has recently been unintentionally used for malicious purposes.

Domain Not Found (or DNS Error)

The recipient's domain name could not be resolved or does not exist.

Solution: check the spelling of the recipient's domain and correct it if necessary; ensure that the domain name and DNS are valid and active.

Mailbox Full

The recipient's inbox is full, preventing new messages from being received.

Solution: Inform the recipient of the situation and ask them to free up space by deleting emails or archiving items.

This cannot happen with email addresses managed by Infomaniak, as they do not have a storage size limit.

Part of their network is on our block list

If the error message (usually displayed after sending an email from an online tool hosted on the server where your website is located, such as an order confirmation from PrestaShop or another online store you manage) contains the following:

host hotmail-com.olc.protection.outlook.com[104.47.30.97] said: 550 5.7.1
Unfortunately, messages from [89.123.456.789] weren't sent. Please contact your Internet service provider since part of their network is on our block list.

Solution: change the email sending method. The sending is unauthenticated and originates from the server with a specific IP address, which is apparently blocked by the recipient for some reason (Infomaniak does not intervene to request unblocking). Therefore, you need to configure the online tool to perform authenticated sending.

Less frequent errors

Deferred - Delayed

The error message Deferred - Delayed - Warning: message still undelivered after 4 hours, Will keep trying until message is 3 days old means that the Infomaniak mail server tried to connect to your recipient's mail server to send the message, but for an undetermined reason, it was unable to do so. This error message specifies that the Infomaniak server will continue to try to send your message during the period specified in the error message. If, after this period, the message has still not been delivered, you will receive a final error message indicating that your message could not be delivered and that your recipient did not receive it.

Greylisting Delay

This Greylisting Delay error occurs when the recipient's mail server uses a technique called "greylisting," which Infomaniak also uses. This technique imposes a temporary delay during the first attempt to deliver a message from an unknown sender. The Mailer Daemon may send an error report indicating that the delivery was delayed due to greylisting. The error may also mention banned sender. You should retry your sending attempt a few minutes/hours later.

Malware message rejected

This Malware message rejected error occurs when the message contains an attachment, a link to a storage site such as Dropbox, or dynamic content, including in the "forwarded" part of the message (in the case of a message forward, for example). Try resending the message from the Infomaniak Mail web app (online service ksuite.infomaniak.com/mail) instead of a messaging application, and remove any content that may be causing this error.

Message Size Exceeded / File Too Large

These Message Size Exceeded / File Too Large errors occur when the size of the message (or attachments) exceeds the limit allowed by the recipient's email server. The Mailer Daemon will return an error report indicating that delivery failed due to the message size exceeding the limit.

Rate Limit Exceeded

This Rate Limit Exceeded error occurs when the sender has exceeded the frequency or volume limit allowed by the recipient's email server. Email servers may impose limits to prevent spam or system overload.

Sender denied: auth guards failed

Please refer to this other guide if you encounter the Sender denied: auth guards failed error.

The message contains a unicode character in a disallowed header

This error, The message contains a unicode character in a disallowed header, may appear if you have inserted a special character, such as a heart ♥, in the body of the email, the subject, or especially in the full name. The risk of rejection by providers is very high: it is best to keep the use of signs, characters, and fonts as simple as possible.