RFC 2821
Commands and codes Error CODE
S: 220
E: 554
S: 250
E: 504, 550
S: 250
E: 552, 451, 452, 550, 553, 503
S: 250, 251 (but see section 3.4 for discussion of 251 and 551)
E: 550, 551, 552, 553, 450, 451, 452, 503, 550
I: 354 -> data -> S: 250 -> E: 552, 554, 451, 452
E: 451, 554, 503
S: 250
S: 250, 251, 252
E: 550, 551, 553, 502, 504
S: 250, 252
E: 550, 500, 502, 504
S: 211, 214
E: 502, 504
S: 250
S: 221
211System status, or system help reply
214Help 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<domain> Service ready
221<domain> Service closing transmission channel
250Requested mail action okay, completed
251User not local; will forward to <forward-path> (See section 3.4)
252Cannot VRFY user, but will accept message and attempt delivery (See section 3.5.3)
354Start mail input; end with <CRLF>.<CRLF>
421<domain> Service not available, closing transmission channel (This may be a reply to any command if the service knows it must shut down)
450Requested mail action not taken: mailbox unavailable (e.g., mailbox busy)
451Requested action aborted: local error in processing
452Requested action not taken: insufficient system storage
500Syntax error, command unrecognized (This may include errors such as command line too long)
501Syntax error in parameters or arguments
502Command not implemented (see section 4.2.4)
503Bad sequence of commands
504Command parameter not implemented
550Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)
551User not local; please try <forward-path> (See section 3.4)
552Requested mail action aborted: exceeded storage allocation
553Requested action not taken: mailbox name not allowed (e.g., mailbox syntax incorrect)
554Transaction failed (Or, in the case of a connection-opening response, "No SMTP service here")

Status code

X.0.0 Other or Undefined Status
There is no additional subject information available.
X.1.[0-8]Addressing Status
The address status reports on the originator or destination address. It may include address syntax or validity. These errors can generally be corrected by the sender and retried.
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.[0-4]Mailbox Status
Mailbox status indicates that something having to do with the mailbox has caused this DSN. Mailbox issues are assumed to be under the general control of the recipient.
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.2.4 Mailing list expansion problem
X.3.[0-5]Mail System Status
Mail system status indicates that something having to do with the destination system has caused this DSN. System issues are assumed to be under the general control of the destination system administrator.
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.[0-7]Network and Routing Status
The networking or routing codes report status about the delivery system itself. These system components include any necessary infrastructure such as directory and routing services. Network issues are assumed to be under the control of the destination or intermediate system administrator.
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.[0-5]Mail Delivery Protocol Status
The mail delivery protocol status codes report failures involving the message delivery protocol. These failures include the full range of problems resulting from implementation errors or an unreliable connection.
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.[0-4]Message Content or Media Status
The message content or media status codes report failures involving the content of the message. These codes report failures due to translation, transcoding, or otherwise unsupported message media. Message content or media issues are under the control of both the sender and the receiver, both of which must support a common set of supported content-types.
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
X.7.[0-7]Security or Policy Status
The security or policy status codes report failures involving policies such as per-recipient or per-host filtering and cryptographic operations. Security and policy status issues are assumed to be under the control of either or both the sender and recipient. Both the sender and recipient must permit the exchange of messages and arrange the exchange of necessary keys and certificates for cryptographic operations.
X.7.0 Other or undefined security status
X.7.1 Delivery not authorized, message refused
X.7.2 Mailing list expansion prohibited
X.7.3 Security conversion required but not possible
X.7.4 Security features not supported
X.7.5 Cryptographic failure
X.7.6 Cryptographic algorithm not supported
X.7.7 Message integrity failure