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A non-delivery report is a message that is sent by the mail server to the sender when the recipient's mailbox is unavailable, does not exist, or the sender/recipient server reports an error due to which it cannot deliver the letter to the recipient within the allotted time.
The text of the letter contains the error text, mailbox address, error code and the reason why the letter could not be delivered.
Detailed error codes can be found at RFC 3463.
We will give examples of the most common errors when emails are not delivered.
# Error: No such user (User not found / No correct recipients / Bad recipient address syntax / Bad address mailbox syntax)
The error indicates that the recipient's email does not exist on the remote server. Check that the recipient's address does not contain extra characters, spaces or symbols. Please confirm the address with the recipient again or try to contact the recipients using alternative methods.
# Error: Message rejected under suspicion of SPAM
If the report contains the line “Message rejected under suspicion of SPAM,” the contents of your email have been detected as spam. Contact the support service of the server that rejected your letter to find out the cause of the problem.
If only spam comes from your mailbox, then some mail servers block receiving letters from it for 24 hours. In this case, the non-delivery report will contain the line “Client host [<IP_host>] blocked using spamsource.. ; see..." or "Blocked by spam statistics - see...".
If you are sure that your computer is not infected with a virus and that spam was not sent from it, please report this to the support service of the server to which the letter was sent via the feedback form, attaching a file of the letter with the report to the letter.
# Error: 550 spam message discarded/rejected
Your email has been blocked by the antispam filter system. To solve this problem, you need to temporarily stop the mailing.
# Error: 550 relay not permitted
There may be several reasons for this error to appear:
- authorization when sending is not enabled in the mail client settings;
- MX servers have incorrect values or for other reasons, which can be clarified with the owners of mx servers.
# Error: Connection timed out / Connection refused / retry time not reached for any host
Such errors usually occur as a result of technical problems on the recipient's mail server. Try waiting a while and trying again, or contact the recipient using alternative methods.
# Error: Mailbox Full or User quota exceeded / Mailbox size exceeded
This problem occurs because the recipient's mailbox is full. Try waiting a while and trying again, or contact the recipient using alternative methods.
# Error: Access to this account (user) mailbox_name is disabled or Mailbox is disabled
This error is issued due to the fact that the recipient's mailbox has been deleted or blocked for lack of use. Try contacting the recipient through alternative means.
# Error: 550 Access from ip address blocked
Most likely your IP address is on one of the blacklists. In this case, contact our technical support for advice.
# The error contains the code "0x800CCC"
To solve this problem, try changing the SMTP port from 25 to port 587 in the settings of your mail program.
If all this does not help, then try using your provider's SMTP server to send.
# Error: Message infected by virus
If the report shows the error “Message infected by virus,” this means that your email was flagged by the recipient's mail server as infected with a virus.
# Error: The recipient has exceeded their message rate limit. Try again later
If the report shows the error “The recipient has exceeded their message rate limit. Try again later", delivery of letters to the recipient's address was temporarily suspended due to the large number of letters arriving in his mailbox per unit of time. Try resending the email in 6 hours.
# Error: Message size exceeds fixed limit (Error: message file too big)
If the report contains the line "Error: message file too big" or "Message size exceeds fixed limit", the attachment you send in the email cannot be accepted by the mail server due to its restrictions.
The maximum size of emails with attachments should not exceed 10 MB.
# Error: too many connections
If the error “Error: too many connections” appears when sending a message, your mail server creates a large number of requests to a third-party mail server per unit of time, which leads to an increase in load and slowdown. Try contacting the administration of your mail service using an alternative method of communication and reporting the problem.
# Error: too many recipients
If the error “Error: too many recipients” appears when sending a message, your message has too many recipients: in the To, Cc and Bcc fields.
