What do the error codes I receive when trying to send emails mean?

Sometimes email programs, such as Outlook, return error messages when trying to send an email. Below are the most common codes.

1. SMTP 421 Service not available, closing transmission channel. This response is sent by the SMTP Server to a remote SMTP Server or to an SMTP Client to indicate that the SMTP Server is not available at that time.
2. SMTP 450 Requested mail action not taken: mailbox unavailable. This response is sent by the server to indicate that a message could not be delivered to the local recipient because your mailbox is busy, possibly receiving another email at that moment.
3. SMTP 451 Requested action aborted: local error in processing. This response is obtained when the SMTP Server encounters an error while processing an SMTP transaction.
4. SMTP 452 Requested action not taken: insufficient system storage. There is not enough space in the account to store the message.
5. SMTP 500 Syntax error, command unrecognized. Indicates that the server could not interpret a command that was sent to the remote SMTP Server or Client.
6. SMTP 501 Syntax error in parameters or arguments. Indicates that the server identified an attempt to send an SMTP command but the parameters associated with it contained a syntax error.
7. SMTP 502. Command not implemented. Indicates that a feature or command requested from the server could not be delivered because it is disabled or not implemented in the SMTP MailEnable connector.
8. SMTP 504 Command parameter not implemented. Indicates that the command sent to the SMTP MailEnable connector contained another command that could not be processed in the same transaction.
9. SMTP 550 Requested action not taken: mailbox unavailable or is not local. Indicates that the recipient address is not hosted on that server and that relay options prevent the message from being forwarded to the correct recipient.
10. SMTP 551 User not local; please try again. Indicates that the recipient specified in the RCPT command is not hosted locally on the server and that relay options prevent the message from being forwarded to the recipient.
11. SMTP 552 Requested mail action aborted: exceeded storage allocation. This response is obtained from the server in response to an RCPT or DATA command when the user has exceeded their mail storage capacity.
12. SMTP 553 Requested action not taken: mailbox name not allowed. Indicates that the format of an SMTP address specified in the RCPT command is incorrect or not properly formatted.
13. SMTP 554 Transaction failed. It is a generic server response when an SMTP transaction fails. This normally occurs when too many processing errors are encountered.
14. SMTP 214 Help message. Information about how to use the receiver or the meaning of a non-standard command; it is a response useful only for the human user.