Exim is a commonly used MTA (mail transfer agent) employed on Unix systems, which is integrated into the cPanel service we offer. It is a versatile email program that provides a wealth of convenient features for managing incoming and outgoing emails. Understanding and being familiar with its status codes can help you resolve basic email issues related to the service. In this article, we’ll attempt to outline the most common response codes and, hopefully, teach you a thing or two about them! The status codes can be divided into four categories.

Status code 2.X.X.X – Successful messages

These codes usually bring a smile to your face and reassure you that everything is fine.

• 211 – This status displays a message regarding the server’s status when a user requests help information.
• 220 – The service is working! This is usually the first message we receive from a working server, and the one we look forward to the most.
• 250 – A response code corresponding to HTTP status code 200 – OK! This status appears every time an email is successfully received or sent.
• 252 – The sender’s response code cannot be verified. The server acknowledges that the user is valid but is unable to verify them. Delivery will take place, or the message will be forwarded to another server capable of verifying the sender.

Status code 3.X.X.X – For information purposes only

• 354 – There is only one status code in this category, and it is triggered by the server receiving the ‘From’ and ‘To’ addresses. It then retrieves the body of the message.

Status code 4.X.X – Persistent/temporary error

These are status codes that may appear from time to time and may persist for an indefinite period, due to temporary server errors occurring on either the sender’s or the recipient’s side.

• 421 – This status code may indicate that a problem has arisen on the server’s end or on the receiving server’s end. It is usually associated with connection issues.
• 422 – This status code means that the recipient’s email inbox is full.
• 431 – This status code indicates that there are storage or memory issues on the recipient’s email server. This may be caused by sending a large number of emails at the same time, or by sending emails containing large attachments. In this case, we recommend that you reduce the number of emails sent simultaneously.
• 450 – This status code indicates that the remote mailbox was unavailable during the delivery attempt. This may occur for various reasons, such as the remote server being offline, a corrupted mailbox, the sender’s IP address being blocked, or it having been added to an RBL (Real-time Blackhole List).
• 452 – This response code means that the requested operation cannot be carried out. You may encounter this if the server is experiencing resource issues, such as memory or storage problems. Another common cause is that the server is attempting to send too many messages.

Status codes 5.X.X – Errors

The most common Exim status codes resulting in rejection. These indicate a problem either on the sender’s or the recipient’s side.

• 503 – This status indicates that you were unable to authenticate yourself with the SMTP server. It usually means that the username or password you entered is incorrect.
• 510 and 511 – Invalid email address. You have entered an address in the ‘TO’, ‘CC’ or ‘BCC’ fields that does not exist on the remote mail server.
• 523 – Every mail server has settings specifying the maximum size of individual messages it will accept. This status code appears if the email you are trying to send exceeds this limit.
• 530 – This could mean three things
  1. You must authenticate yourself with the SMTP server or the recipient’s incoming mail server.
  2. The target server has blacklisted your server.
  3. The recipient’s mailbox does not exist.
• 541 – The recipient’s server rejected the message. This may be caused by an anti-spam program on the recipient’s end.
• 550 – This is likely to be the most common error code you will see. It typically indicates a problem on the recipient’s end and informs you that no such email address exists. This may occur due to the remote server being down, or because the SMTP server’s IP address is blocked by a firewall.
• 552 – Mail operation aborted. This is usually due to the recipient server’s storage capacity being exceeded or the message size exceeding the incoming size limit.
• 554 – This is perhaps the worst status code you can encounter. It informs you that the recipient’s mail server believes your message may be spam, or that your server’s IP address is blacklisted in one or more RBLs.