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 obtained when configuring an email address in a software/email client.

Introduction

• Generally, a mail error message is the result of…
  • … an incorrect configuration of the domain name used with your email,
  • … an issue with your recipient's (mail provider).
• In any case, a delivery failure report (Mailer Daemon or Mail Delivery Subsystem) containing the explanation is sent to the sender.

Searching for and identifying the email error

When your message does not reach your recipient, an initial failure report (Temporary Delivery Failure Report) is sent and specifies that the server will attempt to resend your message several times.

A final delivery failure report (Final Delivery Failure Report or Undeliverable Mail) issued by the recipient's server is sent to the sender (via a passage through the Infomaniak mail server, hence the mention of Infomaniak 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) 

So here the address of your recipient is visibly suspended by their email provider (account administratively disabled).

Sometimes you need to look for the reason for the error message received among its attachments (often a raw 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 correspondent external to Infomaniak seems to be full (Mailbox disk quota exceeded).

Most common cases

SPF Failure

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

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 especially the SPF record if your website is hosted with Wix or another provider.

User Unknown

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

Le serveur de messagerie du destinataire rencontre des problèmes temporaires ou est indisponible.

Solution: wait for some time, then try sending the message again later. If the problem persists, contact the recipient's technical support for help.

Blocked by SPAM Filter (or Content Rejected)

The recipient's mail server has 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 check if the message has been filtered into the junk mail folder.

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

Bad Reputation

The recipient's mail server has 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 https://support.google.com/mail/answer/188131 for more information.

Solution: check the reputation of the domain, determine if one of your email addresses has been recently and involuntarily exploited for malicious purposes, for example.

Domain Not Found (or DNS Error)

Le nom de domaine du destinataire n'a pas pu être résolu ou n'existe pas.

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

La boîte de réception du destinataire est pleine, empêchant la réception de nouveaux messages.

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 since they have no storage size limit.

Part of their network is on our block list

If the error message (usually obtained after sending from an online tool, placed on the server where your website is hosted, for example a order confirmation from Prestashop or another online store that you manage) contains this:

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 is sent from the server with a specific IP address that 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 sendings.

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 contact's mail server to send the message but was unable to do so for an undetermined reason. This error message specifies that the Infomaniak server will continue to attempt to send your message for the period specified in the error message. If, after this delay, the message has still not been delivered, you will receive a final error message informing you that your message could not be delivered and that your contact did not receive it.

Greylisting Delay

This error Greylisting Delay occurs when the recipient's mail server, like Infomaniak, uses a technique called "greylisting" that imposes a temporary delay on the first delivery attempt of a message from an unknown sender. The Mailer Daemon may possibly send an error report indicating that delivery has been delayed due to greylisting. The error may also mention banned sender. You should try resending your message a few minutes/hours later.

Malware message rejected

This error Malware message rejected occurs when the message contains an attachment, a link to a storage site like Dropbox, or dynamic content, including in the "forwarded" part of the message (in the case of a "forward" message, for example). You should try resending the message from the Web app Mail Infomaniak (online service mail.infomaniak.com) instead of a mail application, and remove any content that may cause this error.

Message Size Exceeded / File Too Large

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

Rate Limit Exceeded

This error Rate Limit Exceeded occurs when the sender exceeds the allowed frequency or volume limit set by the recipient's mail server. Mail servers may impose these limits to prevent spam or system overload.

Sender denied: auth guards failed

Refer to this other guide if you encounter an error Sender denied: auth guards failed.

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 like a heart ♥ e.g. in the body of the email, the subject or especially in the full name; the risk of being rejected by providers is very high: it is best to keep the use of signs, characters and fonts as simple as possible.