A list of error codes and thier meaning when sending email and you get a bounce back message:
211 - A system status message.
214 - A help message for a human reader follows.
220 - SMTP Service ready.
221 - Service closing.
250 - Requested action taken and completed. The best message of them all.
251 - The recipient is not local to the server, but the server will accept and forward the message.
252 - The recipient cannot be VRFYed, but the server accepts the message and attempts delivery.
354 - Start message input and end with .. This indicates that the server is ready to accept the message itself (after you have told it who it is from and where you want to to go).
421 - The service is not available and the connection will be closed.
450 - The requested command failed because the user's mailbox was unavailable (for example because it was locked). Try again later.
451 - The command has been aborted due to a server error. Not your fault. Maybe let the admin know.
452 - The command has been aborted because the server has insufficient system storage.
5xx are permanent errors -- The email delivery has failed. The following error messages (500-504) usually tell you that your email client is broken
500 - The server could not recognize the command due to a syntax error.
501 - A syntax error was encountered in command arguments.
502 - This command is not implemented.
503 - The server has encountered a bad sequence of commands.
504 - A command parameter is not implemented.
550 - The requested command failed because the user's mailbox was unavailable (for example because it was not found, or because the command was rejected for policy reasons).
551 - The recipient is not local to the server. The server then gives a forward address to try.
552 - The action was aborted due to exceeded storage allocation.
553 - The command was aborted because the mailbox name is invalid.
554 - The transaction failed.
