The RFC's do define the first three digits of error codes but the software or mail server itself can define anything after the first three digits for their own use. Perhaps the time is coming that people can be given some uniform codes they can understand. Though the problems are bigger than they may appear at first glance. We have put together some basic codes as a guideline to help everyone have some idea as to what the error might mean.
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: The 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.
554: Transaction failed or Permanent Failure - The server sending your mail server does not have a reverse DNS entry.
1. Helo command rejected: Access denied;
2. Recipent user is "Over Quota"
3. You do not have permission to send to this recipient.
557: Too many duplicate messages: Resource temporarily unavailable - Indicates (probable) that there is some kind of anti-spam system on the mail server.