New enhanced ESMTP Status/Error Codes
and descriptions defined by RFC Protocol

200  Nonstandard success response
see http://www.faqs.org/rfcs/rfc876.html

211 System status or system help reply 

214 Help message - A help message for human reader follows. (Information on how to use the receiver or the meaning of a particular non-standard command)
 

220 <domain> Service ready - Confirming the connection to your ISP mail server.
 

221 <domain> Service closing transmission channel
To stop sending email after a QUIT signal is send
to your ISP mail server.

251 User not local, will forward to <forward-path>
Your message to a specified email address is not local
to the mail server, but it will accept and forward the message to a different recipient email address.

252 Recipient cannot be verified - but mail server accepts the message and attempts delivery.

354 Start mail input and end with <CRLF> <CRLF>
Indicates mail server is ready to accept the message or instruct your mail client to send the message body after the mail server have received the message headers.
 

421 <domain> Service not available, closing transmission channel - This may be a reply to any command if the service knows it must shut down.
 

450 Requested mail actions not taken: mailbox unavailable - Your ISP mail server indicates that an email address does not exist or the mailbox is busy.

It could be the network connection went down while sending, or it could also happen if the remote mail server does not want to accept mail from your for some reason i.e. (IP address, From address, Recipient, etc.)
 

451 Requested actions aborted: local error in processing
Your ISP mail server indicates that the mailing has been interrupted, usually due to overloading from too many messages or transient failure is one in which the message sent is valid, but some temporary event prevents the successful sending of the message.
Sending in the future may be successful.
 

452 Requested actions not taken: insufficient system storage
Your ISP mail server indicates, probable overloading from too many messages and sending in the future may be successful.
 

452 too many messages' error - Some mail servers have the option to reduce the number of concurrent connection and also the number of messages sent per connection.

If you have a lot of messages queued up it could go over the max number of messages per connection.

To see if this is the case you can try submitting only a few messages to that domain at a time and then keep increasing the number until you find the maximum number accepted by the server.

500 Syntax error, command unrecognized
This may include errors such as command line too long.
 

501 Syntax error in parameters or arguments - Indicates possible poor (noisy dialup) or an intermittent drop in network line connection that caused your mail client to send erroneous command to the mail server.
 

502 Command not implemented - Indicates that your ISP mail server did not recognized a command sent.
 

503 Server encountered bad sequence of commands - Indicates (probable) that your ISP mail server did not recognized a command sent that is erroneous.

Some temporary event prevents the successful sending of the message or an intermittent drop in network line connection that caused your mail client to send erroneous command and sending in the future may be successful.
 

504 Command parameter not implemented - Indicates that your ISP mail server did not recognized a command sent.

521 <domain does not accept mail or closing transmission channel You must be pop-authenticated before you can use this SMTP server and you must use your mail address for the Sender/From field.
 

530 Access denied (???a Sendmailism)
 

550 Requested actions not taken, mailbox unavailable - Indicates that your recipient's email address was not recognized by your ISP mail server or (mailbox not found or cannot access it).
 

550 Relaying prohibited or Not local host… not a gateway or Unable to relay for…    or user’s mailbox unavailable
Sending an email to recipients outside of your domain are not allowed or your mail server does not know that you have access to use it for relaying messages and authentication is required. Or to prevent the sending of SPAM some mail servers will not allow (relay) send mail to any e-mail using another company’s network and computer resources.
 

550 This address is not allowed or Requested action not taken: mailbox unavailable - Seems like the setting of the “From Address” are incorrect and/or an attempted was made to deliver but there was a non fatal error and it will be retried and/or some change to the message destination must be made for successful delivery.
 

551 User not local, please try <forward-path> or Invalid Address: Relay request denied - Indicates that the recipient's email address have changed and your ISP mail server is forwarding it back to you and/or your ISP SMTP mail server does not accept email when neither the sender nor the recipient is a local user--this feature was implemented to protect the mail server from being used by spammers to relay their messages by using another company’s network and computer resources.
 

552 Requested mail actions aborted: exceeded storage allocation - ISP mail server indicates, probable overloading from too many messages.
 

553 Denied. Requested action not taken: mailbox name not allowed or bad command format - (E.g., mailbox syntax incorrect)  Some mail servers have the option to reduce the number of concurrent connection and also the number of messages sent per connection.

If you have a lot of messages queued up (being sent) for a domain, it could go over the maximum number of messages per connection and/or some change to the message and/or destination must be made for successful delivery.
 

554 Transaction failed or Permanent Failure
 A permanent failure is one which is not likely to be resolved by resending the message in its current form and some change to the message and/or destination must be made for successful delivery.
 

557 Too many duplicate messages: Resource temporarily unavailable - Indicates (probable) that there is some kind of anti-spam system on the mail server.
 

572 Relay not authorized or Not local host… not a gateway - Not allowed to send email to recipients outside of your domain. To prevent SPAM many mail server will not allow you to send mail to any ISP SMTP mail server when either the sender or recipient is a local user.

This feature is implemented to protect the mail server from being used by spammers to relay their messages using another company’s network and computer resources. Some mail servers required you to log in before sending messages.

Next Page 3/3 Request for Comment (RFC)
A document outlining a standard and proposed standard to an existing standard.

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