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The ESA is fully-compliant with the original Simple Mail Transfer Protocol (SMTP) specification of RFC 821. The complete list of message response codes are listed in RFC 821.
The 400 series of codes indicate non-fatal errors or responses in the SMTP conversation. Such errors are often referred to as soft-bounces, as unless otherwise instructed, the transmitting mail server will re-attempt delivery to the intended receiver at a later time. Soft-bounces can be caused by a variety of things ranging from a mail account exceeding its quota to SMTP hosts policy limits imposed upon a particular sender.
The 500 series of codes are also error codes. Unlike the soft-bounces of the 400 series however, the 500 denote a complete failure on the part of the receiving server to accept a message for delivery. These failures, referred to as hard-bounces, are typically caused by such issues as malformed or unknown mail-to addresses, a failed reverse DNS check, or other receiver host policy limits.
It is important to note that depending upon the policy set for the transmitting mail server, a soft-bounce may eventually become a hard-bounce
Here is a list of the most common SMTP response codes:
211 System status, or system help reply
214 Help messageInformation on how to use the receiver or the meaning of aparticular non-standard command; this reply is useful onlyto 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 <CRLF>.<CRLF>
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: error in processing
452 Requested action not taken: insufficient system storage
453 Too Many Recipients
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