| code | meaning |
| 211 | System status or system help reply |
| 214 | Help message |
| 220 | Service ready |
| 221 | Service closing transmission channel |
| 250 | Requested mail action okay |
| 251 | User not local will forward to |
| 252 | Cannot VRFY user but will take message and attempt delivery |
| 253 | messages pending for node started |
| 354 | Start mail input end with |
| 355 | Octet-offset is the transaction offset |
| 421 | Service not available |
| 432 | A password transition is needed |
| 450 | Requested mail action not taken: mailbox unavailable |
| 451 | Requested action aborted: local error in processing |
| 452 | Requested action not taken: insufficient system storage |
| 453 | You have no mail |
| 454 | TLS not available due to temporary reason |
| 458 | Unable to queue messages for node node |
| 459 | Node node not allowed: reason |
| 500 | Syntax error |
| 501 | Syntax error in parameters or arguments |
| 502 | Command not implemented |
| 503 | Bad sequence of commands |
| 504 | Command parameter not implemented |
| 521 | Machine does not accept mail |
| 530 | Must issue a STARTTLS command first |
| 550 | Requested action not taken: mailbox unavailable |
| 551 | User not local please try forwardpath |
| 552 | Requested mail action aborted: exceeded storage allocation |
| 553 | Requested action not taken: mailbox name not allowed |
| 554 | Transaction failed |
Enhanced Error Codes
x.1.0 Other address status
x.1.1 Bad destination mailbox address
x.1.2 Bad destination system address
x.1.3 Bad destination mailbox address syntax
x.1.4 Destination mailbox address ambiguous
x.1.5 Destination mailbox address valid
x.1.6 Mailbox has moved
x.1.7 Bad sender’s mailbox address syntax
x.1.8 Bad sender’s system address
x.2.0 Other or undefined mailbox status
x.2.1 Mailbox disabled, not accepting messages
x.2.2 Mailbox full
x.2.3 Message length exceeds administrative limit
x.3.0 Other or undefined mail system status
x.3.1 Mail system full
x.3.2 System
not accepting network messages
x.3.3 System not capable of selected features
x.3.4 Message too big for system
x.4.0 Other or undefined network or routing status
x.4.1 No answer from host
x.4.2 Bad connection
x.4.3 Routing server failure
x.4.4 Unable to route
x.4.5 Network congestion
x.4.6 Routing loop detected
x.4.7 Delivery time expired
x.5.0 Other or undefined protocol status
x.5.1 Invalid command
x.5.2 Syntax error
x.5.3 Too many recipients
x.5.4 Invalid command arguments
x.5.5 Wrong protocol version
x.6.0 Other or undefined media error
x.6.1 Media not supported
x.6.2 Conversion required and prohibited
x.6.3 Conversion required but not supported
x.6.4 Conversion with loss performed
x.6.5 Conversion failed
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