3xx reply
An SMTP reply starting with 3, meaning the server accepted the command and is waiting for more input, as with 354 after DATA.
What it means
RFC 5321 calls these positive intermediate replies: the command was accepted, but the action waits for more information from the client. Two are common. 354 answers the DATA command and invites the message, which ends with a line holding a single dot. 334 is used by SMTP AUTH (RFC 4954) to send a challenge during login, which the client answers in base64.
A 3xx reply never completes a step on its own: the client must send what was asked for, and the step ends with a later 2xx, 4xx or 5xx reply.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321, section 4.2.1: Reply Code Severities and Theoryrfc-editor.org
- RFC 4954: SMTP Service Extension for Authenticationrfc-editor.org
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