SMTP/ESMTP error message description
211 System status, or system help reply
214 Help 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 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
(This may be a reply to any command if the service knows it must shut down)
450 Requested mail action not taken: mailbox unavailable
(E.g., mailbox busy)
451 Requested action aborted: local error in processing
452 Requested action not taken: insufficient system storage
500 Syntax error, command unrecognized
(This may include errors such as command line too long)
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
(E.g., mailbox not found, no access)
551 User not local; please try
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
(E.g., mailbox syntax incorrect)
554 Transaction failed

5.7.1 is a enhanced 551 error code

For complete list of all Error codes, it can be found here

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by ThirdShifter