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In addition, any EHLO keyword value starting with an upper or lower case "X" refers to a local SMTP service extension used exclusively through bilateral agreement. Keywords beginning with "X" MUST NOT be used in a registered service extension. Conversely, keyword values presented in the EHLO response that do not begin with "X" MUST correspond to a standard, standards-track, or IESG-approved experimental SMTP service extension registered with IANA. A conforming server MUST NOT offer non-"X"-prefixed keyword values that are not described in a registered extension.

Additional verbs and parameter names are bound by the same rules as EHLO keywords; specifically, verbs beginning with "X" are local extensions that may not be registered or standardized. Conversely, verbs not beginning with "X" must always be registered.

SMTP Error Messages

211 system status message
214 help message for a human reader follows
220 service ready
221 service closing
250 requested action taken and completed  *best message of them all*
251 recipient is not local to  server, but it will accept and forward  message
252 recipient cannot be VRFYed, but  server accepts  message and attempts delivery
354 Start message input and end with <CRLF><CRLF>  server is ready to accept  message itself
421 service is not available and  connection will be closed
450 requested command failed because  user's mailbox was unavailable (for example since it was loced)
451 command has been aborted due to a server error Not your fault
452 command has been aborted because  server has insufficient system storage
500 server could not recognize  command due to a syntax error
501 A syntax error was encountered in command arguments
502 This command is not implemented
503 server has encountered a bad sequence of commands
504 A command parameter is not implemented
550 Command failed because user's mailbox was unavailable (it was'nt found or command was rejected for policy reasons)
551 recipient is not local to  server  server n gives a forward address to try
552 action was aborted due to exceeded storage allocation
553 command was aborted because  mailbox name is invalid
554 transaction failed

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