425 Unordered Collection - existed only in draft
444 No Response - Nginx 449 Retry With - Microsoft 450 Blocked by Windows Parental Controls - Microsoft 499 Client Closed Request - Nginx
506 Variant Also Negotiates - RFC 2295; experimental
508 Loop Detected - RFC 5842; experimental 509 Bandwidth Limit Exceeded - Apache bw/limited extension 510 Not Extended - RFC 2774; experimental
Non-authentication POP3 protocol error (reply code “-ERR”, except authentication).
Represents an SMTP command syntax error (error code 500)
Represents a fatal SMTP error (error code 5xx, except for 500)