This is a list of Simple Mail Transfer Protocol (SMTP) response status codes.
A "Basic Status Code" SMTP reply consists of a three digit number (transmitted as three numeric characters) followed by some text.
The first digit denotes whether the response is good, bad, or incomplete: The second digit encodes responses in specific categories: The Basic Status Codes have been in SMTP from the beginning, with RFC 821 in 1982, but were extended rather extensively, and haphazardly so that by 2003 RFC 3463 rather grumpily noted that: "SMTP suffers some scars from history, most notably the unfortunate damage to the reply code extension mechanism by uncontrolled use."
[1] The Internet Assigned Numbers Authority (IANA) maintains the official registry of these enhanced status codes.
This list is not exhaustive, and the actual text message (outside of the 3-field Enhanced Status Code) might be different.