2xx/3xx class - Success
They are SMTP protocols successfull answers
4xx class - Temporary Errors
||200 Non standard success response
Non standard success response
||211 System status, or system help reply
System status, or system help reply
||214 Help message
||220 Service ready
||221 Service closing transmission channel
Service closing transmission channel
||250 Requested mail action taken and completed
Your ISP mail server have successfully executes a command and the DNS is reporting a positive delivery.
||251 User not local: will forward to
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
Recipient cannot be verified but mail server accepts the message and attempts delivery
||354 Start mail input and end with .
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.
Those codes are temporary error message. They are used to tell client sender that
an error occured but he can try to solve it but trying again, so in most cases, clients that
receive such codes will keep the mail in their queue and will try again later.
5xx class - Permanent Errors
||421 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 action not taken: mailbox busy or access denied
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 you for some reason i.e. (IP address, From address, Recipient, etc.)
||451 Requested mail action aborted: 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 mail action not taken: insufficient system storage
Your ISP mail server indicates, probable overloading from too many messages and sending in the future may be successful.
||453 Too many messages
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.
This are permanent error codes. Mail transfer is definitly a failure. No other try will be done.
||500 Syntax error, command unrecognized or command line too long
||501 Syntax error in parameters or arguments
||502 Command not implemented
||503 Server encountered bad sequence of commands
||504 Command parameter not implemented
||521 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 mail action not taken (Relaying not allowed, Unknown recipient user, ...)
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.
||551 User not local: please try or Invalid Address: Relay request denied
||552 Requested mail action aborted: exceeded storage allocation
ISP mail server indicates, probable overloading from too many messages.
||553 Requested mail action not taken: mailbox name not allowed
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 Requested mail action rejected: access denied
||557 Too many duplicate messages
Resource temporarily unavailable Indicates (probable) that there is some kind of anti-spam system on the mail server.