Signal & Syntax · Email deliverability

What Are SMTP Status Codes?

SMTP status codes are three-digit mail server replies. 2xx means success, 4xx a temporary failure and 5xx a permanent failure.

Also calledSMTP reply codes · SMTP error codes

Example

250 OK · 450 try later · 550 mailbox unavailable

SMTP Status Codes in Practice

Code 250 accepts the command. Code 550 rejects the mailbox as unavailable. Codes 421 and 450 ask the sender to retry later. Enhanced codes such as 5.1.1 add detail: 5.1.1 means the mailbox does not exist.

Where ScoreMachine Fits

score.email.validation

smtp_status_code

smtp_status_code returns the code behind each deliverability verdict.

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Related Terms

SMTP SMTP (Simple Mail Transfer Protocol) is how mail servers send email to each other. Hard bounce A hard bounce is a permanent email delivery failure. Soft bounce A soft bounce is a temporary email delivery failure. MX record An MX (mail exchanger) record is the DNS entry naming a domain's mail servers.

Read the Guide

Guide

MX Records, SMTP Codes and Bounces Explained

How an email finds a mailbox, and how the server answers. Read the guide →

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