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SMTP is responsible for sending your messages. If you receive an SMTP error message, it means that your emails have not been sent. You need to know what caused this problem. All SMTP codes consist of three numbers, for example, 550, 221, 354, etc. Not all of them imply some type of error. To understand the meaning of error codes you need to know that each digit (first, second and third) has its own meaning.

The first digit shows you if your team has been accepted and processed. There are five different meanings:

The mail server has accepted the command, but is not taking any action yet. A confirmation message is required.
The mail server completed the task successfully without errors.
The mail server understood the request, but requires further information to complete it.
The mail server has experienced a temporary failure. If the command is repeated without change. Try again, it can help!
The mail server encountered a fatal error. Your request could not be processed.
If you see codes starting with 4, and 5, this means that your message will not be sent until you fix the problem.

The second number means:

Syntax error
Informational response (for example, to a REQUEST FOR REFERENCE INFORMATION)
This figure refers to the connection status.
This figure refers to the state of the mail server.
The last digit of the code shows the details of the transmission status.

Here is a list of the most important SMTP error codes:

421 Service is not available, the transmission channel is closed (This can be a response to any command if the service {service} knows that this should close)

450 Required mail actions not taken: the mailbox is unavailable (for example, the mailbox is busy)

451 Requested action aborted: processing error

452 Required actions not taken: insufficient system resources

500 Syntax error, incorrect command (This may include errors such as too long command line)

501 Syntax error in parameters or variables

502 non-existent team

503 Incorrect command sequence

504 Command parameter not implemented

550 Required actions not taken: mailbox unavailable (for example, mailbox not found, no access)

551 User is not local; try again

552 Required mail activity failed: memory allocation exceeded

553 Required actions not taken: mailbox name, invalid (for example, the mailbox syntax is incorrect)

554 Data transfer failed

Other codes that provide you with useful information about what is happening with your messages:

211 System status, or system help answer

214 Help Message (Information on how to use the recipient or the value of a specific non-standard command; this answer is only useful to the human user)

220 Ready for service

221 Served final transmission channel

250 Required mail activity completed

251 User is not local; message forwarded

354 Begin typing a message. Complete the message entry. (point).