The most common reasons for non-delivery of messages are:
Returns that prevent message delivery
Here are the phrases that prevent further delivery of the message:
- 511 – Invalid username
The user with the specified address does not exist, and his account has probably been deleted. Therefore, the message cannot be delivered. - 512 – There is no such domain
The indicated domain is not the address of the mail system. This means that either such a domain does not exist, or you cannot specify a mail server for it, which prevents message delivery. - 513 – Unknown email address
The indicated email address is invalid because no such user exists, there is no such mail server, or the address is syntactically incorrect. Therefore, it is not possible to deliver the message. - 517 – Invalid sender address
The sender’s address is incorrect according to the postal system, which prevents further transmission of messages. - 521 – Inactive mailbox
The mailbox exists, but it is disabled and does not accept messages. Most often, the mailbox is disabled by the administrator, which means that message delivery is impossible. - 523 – Message too long
The message sent is longer than the maximum limit per message set by the recipient’s server administrator. Therefore, the message cannot be delivered. - 530 – Undefined mail system error
When sending a message, the addressee’s mail system signaled an error, but did not specify the reason for non-delivery. In this situation, delivery of the message is impossible. - 532 – The system does not accept external messages
The addressee’s mail system exists, but it does not accept messages sent from outside. It is possible to send messages only from the same server (message delivery impossible). - 535 – Mail system configured incorrectly
The message was not delivered because the addressee’s mail system has configuration errors (message delivery impossible). - 540 – Undefined network connection error
A hard-to-define error occurred in the communication between the sender’s and recipient’s mail systems during delivery (message delivery impossible). - 541 – No response from the mail server
The mail system is not responding. While trying to send a message, the mail server did not respond because it was turned off, busy or for some other reason was unable to establish a connection (message delivery impossible). - 542 – Transmission error
There were problems while sending the message. The connection between mail systems was established, but it was not possible to complete it (message delivery impossible). - 543 – DNS server problem
When sending a message, the system could not establish a connection with the DNS server serving the addressee’s domain and stopped further sending attempts (message delivery impossible). - 544 – Unable toroute
When sending a message, the mail server had a problem pointing to the next server (it was unable to get the necessary information from the DNS server about the next server) on the routing path (message delivery impossible). - 546 – Routing loops detected
Message routing problems. A routing loop caused the message to be rejected because it was forwarded too many times to successive servers or the routing tables were incorrect (message delivery impossible). - 547 –The delivery time has been exceeded
The maximum time for sending a message has expired (probably due to the occurrence of other errors) and it has not been delivered. In this case, the mail system does not retry sending the message (message delivery impossible). - 550 – Undefined mail protocol error
An undefined mail protocol error (message delivery impossible) occurred while sending a message. - 552 – Syntax error
When sending a message, an attempt was made to call a non-existent command or a grammatically incorrect command. Most often the cause of the error is an incorrectly configured mail server of the addressee (delivery of the message impossible). - 554 – Invalid command argument
Incorrect mail protocol command argument. Most often the cause of the error is an incorrectly configured mail server of the addressee (message delivery impossible). - 571,594 – Message delivery refused
The message was rejected by the receiving server, due to its own message filtering policy or due to the sender’s or recipient’s address (message delivery impossible). - 591 – Message blocked due to content
Message contains forbidden content e.g. forbidden words, attachments (message delivery impossible). - 592 – The message contains HTML script
The sent message contains HTML content in which a script (e.g. JavaScript) has been embedded and the addressee’s server does not allow such content (message delivery impossible). - 574 – Authentication/verification required
The receiving server has been misconfigured.
Other returns
These are phrases that enable the delivery of messages:
- 421 – Temporarily deactivated mailbox
The mailbox exists, but has been temporarily deactivated by the mail system administrator and does not accept messages. If it is reactivated, the message will be delivered. - 422 – Overflowing addressee mailbox
The message did not reach the addressee because there is no space for new messages. The mail system will try to resend the message. If the user deletes older messages and frees up space, the message will be delivered. - 441 – No response from mail server
The addressee’s mail server did not respond when sending the message because it was busy or unable to establish a connection. - 443 – Routing server error
When sending a message, the system could not connect to the DNS server serving the address. - 445 – Addressee’s mail system overloaded
The addressee’s mail system was overloaded and unable to receive messages. - 447 – Delayed delivery
A timeout has been exceeded while sending a message. - 470 – Undefined client-side security error
There were hard-to-define security problems on the addressee’s server side, making it difficult to deliver the message. - 530 – Unidentified mail system error
When sending a message, the addressee’s mail system signaled an error, but did not specify the reason for non-delivery.
We hope that the above information will help you understand the reasons for non-delivery of messages. If you have any questions, we are at your service.