200 Codes
- 211 - System status / system help reply
- 214 - Help message
- 220 - Domain service ready
- 221 - Domain service closing transmission channel
- 250 - Requested mail action completed and OK
- 251 - Not Local User, forward email to forward path
- 252 - Cannot Verify user, will attempt delivery later
- 253 - Pending messages for node started
300 Codes
- 354 - Start mail input; end with .
- 355 - Octet-offset is the transaction offset
400 Codes
- 421 - Domain service not available, closing transmission channel
- 432 - Domain service not available, closing transmission channel
- 450 - Requested mail action not taken: mailbox unavailable. request refused
- 451 - Requested action aborted: local error in processing Request is unable to be processed, try again
- 452 - Requested action not taken: insufficient system storage
- 453 - No mail
- 454 - TLS not available due to temporary reason. Encryption required for requested authentication mechanism
- 458 - Unable to queue messages for node
- 459 - Node not allowed: reason
500 Codes
- 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
- 510 - Check the recipient address
- 512 - Domain can not be found. Unknown host.
- 515 - Destination mailbox address invalid
- 517 - Problem with senders mail attribute, check properties
- 521 - Domain does not accept mail
- 522 - Recipient has exceeded mailbox limit
- 523 - Server limit exceeded. Message too large
- 530 - Access Denied. Authentication required
- 531 - Mail system Full
- 533 - Remote server has insufficient disk space to hold email
- 534 - Authentication mechanism is too weak. Message too big
- 535 - Multiple servers using same IP. Required Authentication
- 538 - Encryption required for requested authentication mechanism
- 540 - Email address has no DNS Server
- 541 - No response from host
- 542 - Bad Connection
- 543 - Routing server failure. No available route
- 546 - Email looping
- 547 - Delivery time-out
- 550 - Requested action not taken: mailbox unavailable
- 551 - User not local; please try forward path
- 552 - Requested mail action aborted: exceeded storage allocation
- 553 - Requested action not taken: mailbox name not allowed
- 554 - Transaction failed
