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Here is the list of most important SMTP error codes:

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 unavailable (E.g., mailbox busy)
451: Requested action aborted: local error in processing
452: Requested action not taken: insufficient system storage
500: Syntax error, command unrecognized (This may include errors such as command line too long)
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 (E.g., mailbox not found, no access)
551: User not local; please try
552: Requested mail action aborted: exceeded storage allocation
553: Requested action not taken: mailbox name not allowed (E.g., mailbox syntax incorrect)
554: Transaction failed

The other codes that provide you with helpful information about what's happening with your messages are:

211: System status, or system help reply
214: Help message (Information on how to use the receiver or the meaning of a particular non-standard command; this reply is useful
only to the human user)
220: Service ready
221: Service closing transmission channel
250: Requested mail action okay, completed
251: User not local; will forward to
354: Start mail input; end with . (a dot)