What is an SMTP Error Code?

When you sent a message from a mail client via the SMTP server, the server will reply with a specific response code. This response is a normal part of the email sending process and is an essential step in getting your email to the sent.

An SMTP(Simple Mail Transfer Protocol), a three-digit code response code between two servers to communicate throughout the email sending process

These types of response codes can be useful to verify the errors and debug it, like 250, or it can indicate that there was a communication error.

If there is a communication error, you will receive an SMTP error code — a message that indicates the specific problem; like the end of server communication, which is error code 550 (The request failed).

Positive SMTP Response Codes

Generally, Positive SMTP response codes start with regular 2 and 3, that doesn’t indicate an error.

SMTP 200 Response Codes

• 220 – SMTP server is ready to sent emails
• 221 – SMTP server is closing its transmission channel
• 250 – requested action is accepted and triggered (Good news!)

SMTP 300 Response Codes

A response code starting with 3 indicates the request is identified, but the server needs more information to complete the task.

• 334 – SMTP authentication credentials is Invalid.
• 354 – Waiting for the email message contents

SMTP Server Error Messages

SMTP 400 Error Codes

Error code 4 means the server is facing a temporary failure.if the server tried again, the process might be completed. Error codes starting with 4 are typical errors related to the mail system(receiver).

Note: Response codes starting with 4 and 5 indicates that an error has occurred onto the system.

• 421 – Email service is unavailable. Wait for some interval and try again.
• 450 – user’s mailbox was unavailable hence requested action was not taken
• 451 – Email not sent because of server error
• 452 – Storage Full and command aborted
• 455 – Server cannot trigger command this time

SMTP 500 Error Codes

The most prominent SMTP error code indicator starts with error code, 5.  If you notice this type of failure then, this is a permanent. Here are few examples:

• 500 – Syntax error, Server could not recognize the command
• 501 – Syntax error, Invalid arguments or parameters
• 502 – Command cannot be triggered
• 503 – Invalid sequence of server commands
• 541 – Receipient address rejected the email
• 550 – Requested command failed due to the receipient mail server was unavailable or the server rejected the email because of spam high probability
• 551 – Recipient address not found on the server
• 552 – Recipient address does not have adequate storage, hence message was not sent
• 553 – Command aborted because the mailbox name does not exist.
• 554 – The transaction failed. No further details are given.

How Do You Fix an SMTP Error?

These are few SMTP error codes, and there are many more that you will likely come across. Hence, most high volume email senders will work with email service providers.

Step 1

The solution was to enable outgoing SMTP from the server settings.

On servers running cPanel’s WHM, this is located under WHM’s “Tweak Settings” section.

The option is to enable/disable – choose disable.

Caveat: Making this change will redirect outgoing SMTP connections allow accounts to make direct connections, which may increase your odds of getting your server blacklisted.

Step 2

Restrict outgoing SMTP to root, Exim, and mailman (FKA SMTP Tweak)

Login to CPanel > Tweak Settings > All> “Restrict outgoing SMTP to root, Exim, and mailman (FKA SMTP Tweak) and switch it from “On” to “Off.”

for my case, I used the shell (ssh) to fix this :

/scripts/smtpmailgidonly off

Output

“SMTP Mail protection has been disabled. All users may make outbound SMTP connections.”