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.

Preamble

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

Finding 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 pass through the Infomaniak mail server, hence the mention of Infomaniak at the beginning of most error messages you may receive) to inform 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 correspondent is apparently 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 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 correspondent outside Infomaniak seems 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 type record if your website is hosted on 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; make sure to use 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 a certain amount of time, 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 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 verify if the message was filtered into the spam 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 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 do not have storage size limits.

Part of their network is on our block list

If the error message (obtained in principle following a send operation performed from an online tool, placed on the server where your website is located, for example a Prestashop order confirmation or other online store 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 comes 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 sends.

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 correspondent's mail server to send the message but failed for an undetermined reason. This error message specifies that the Infomaniak server will continue to attempt to send your message during 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 correspondent did not receive it.

Greylisting Delay

This error Greylisting Delay occurs when the recipient's mail server, like Infomaniak, uses a technique called "greylisting" which imposes a temporary delay on the first attempt to deliver a message from an unknown sender. The Mailer Daemon may eventually send an error report indicating that delivery has been delayed due to greylisting. The error may also mention banned sender. You should try sending your message again 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 message "forward", for example). You should try resending the message from the Infomaniak Mail Web app (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 has exceeded the frequency or volume limit allowed by the recipient's mail server. Mail servers may impose 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 ♥, for example, 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.