SMTP / FTP / IMAP Codes
Searchable reference of SMTP, FTP and IMAP response codes.
Overview
The SMTP / FTP / IMAP codes reference is a searchable list of every response code each of these three protocols defines, with the standard reason phrase and a plain-language note on what triggers it. Type 550 to see SMTP "Requested action not taken: mailbox unavailable", 421 for "Service not available, closing transmission channel", or 220 for the typical FTP greeting.
Mail administrators reading a bounce log, FTP/SFTP support staff diagnosing transfer failures, and developers writing protocol clients all need a quick SMTP/FTP/IMAP code lookup. Long-tail keywords covered: SMTP 550 vs 552 difference, what does FTP 425 mean, and IMAP NO vs BAD response codes.
How it works
SMTP (RFC 5321) and FTP (RFC 959) use three-digit response codes. The first digit is the class — 1xx informational, 2xx success, 3xx need-more-info, 4xx transient failure, 5xx permanent failure. The second and third digits classify the subject and detail. IMAP (RFC 9051) uses tagged words instead — OK, NO, and BAD — paired with a response code in brackets for machine-readable detail.
Bounces from mail servers usually include both an SMTP code and an enhanced status code (RFC 3463), like 550 5.1.1 User unknown. The enhanced code adds a class/subject/detail trio that makes the cause more precise than the legacy three-digit code alone.
Examples
- SMTP
220— service ready (greeting). - SMTP
421— service unavailable, closing transmission (transient). - SMTP
550 5.1.1— mailbox does not exist (permanent). - FTP
425— can't open data connection (often firewall or NAT trouble).
FAQ
Is a 4xx code recoverable?
Yes — by definition, 4xx is transient. The sender should retry later. 5xx is permanent and should not be retried with the same parameters.
What is an enhanced status code?
A second, more specific code defined in RFC 3463 that augments the legacy SMTP three-digit code. 5.1.1 means "bad destination mailbox address", which is more actionable than the bare 550.
Why does IMAP have only three response words?
OK, NO, and BAD cover the protocol's vocabulary cleanly: success, command failed, command malformed. Detail comes from optional bracketed response codes and from the text following the word.
Are response phrases standardised?
Codes are. Phrases are recommended but not normative — implementations vary. Always parse the numeric code, never the text.