Here are the meaning of SMTP status codes.
| Status Code | Description |
| 211 | System status, or system help reply |
| 214 | Help message |
| 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 "." |
| 421 | Service not available, closing transmission channel |
| 450 | Requested mail action not taken: mailbox unavailable |
| 451 | Requested action aborted: local error in processing |
| 452 | Requested action not taken: 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 |
| 551 | User not local; please try <....> |
| 552 | Requested mail action aborted: exceeded storage allocation |
| 553 | Requested action not taken: mailbox name not allowed |
| 554 | Transaction failed |
原文：http://www.cnblogs.com/oskb/p/4150429.html