Email error messages usually mean what they say. For example, if your bounce message says, "mail box is full," the mailbox of the person you're
trying to contact is probably full. If the message says "Unknown "local part" in domain "yourdomain.com"" then there is probably something wrong
with the email address that has been entered in the TO: field, etc. Following are the most common error numbers with SMTP error messages
attached to them, which may vary from server to server:
SMTP Error Messages:
Error # Error Message (may vary)
421 Service not available, closing transmission channel
450 Mailbox unavailable
451 Local error in processing
452 Insufficient system storage
500 Syntax error, command unrecognized
501 Syntax error in parameters or arguments
502 Command not implemented
503 Bad sequence of commands
504 Command parameter not implemented
550 Requested action not taken: mailbox unavailable (or Relaying Denied)
551 User not local
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
554 Transaction failed
9100 Not enough memory
9101 A task is already in progress
9102 No recipient was specified
9103 Aborted by user
9104 Invalid email address
9105 Invalid mail server address
9106 Invalid user name
9107 Invalid password
9108 Mail server is not responding
9109 Invalid reply from the server
9111 One or more e-mails were undeliverable | 
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