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ProtocolCodes
421
SMTP

421 Service Not Available

The SMTP service is not available and the server is closing the transmission channel. This may be sent as a greeting or during a session if the server needs to shut down.

On this page

When You See This Code

When connecting to a mail server that is overloaded, undergoing maintenance, or shutting down. The server cannot handle your request right now.

How to Fix

Retry the connection after a delay. If the issue persists, check server status and resources (CPU, memory, disk). Rate limiting or greylisting may also trigger this response.

Specification

View RFC / Specification →

Comparisons

SMTP 421 Service Not Available vs 521 Host Does Not Accept Mail

Compare With

211 System Status 214 Help Message 220 Service Ready 221 Closing Connection 235 Authentication Successful 250 OK 251 User Not Local 252 Cannot Verify User 334 Server Challenge 354 Start Mail Input 450 Mailbox Unavailable 451 Local Error

Guides

Debugging SMTP Delivery Failures: 4xx Temporary vs 5xx Permanent Debugging & Troubleshooting
SMTP Reply Codes: What Your Email Server Is Telling You Email Delivery
Improving Email Deliverability: SPF, DKIM, and DMARC Setup Email Delivery
Diagnosing Email Bounces: Soft (4xx) vs Hard (5xx) Email Delivery
SMTP Authentication Methods Explained Email Delivery
Email Header Analysis: Tracing Delivery Path Email Delivery
Bulk Email Delivery: Avoiding the Spam Folder Email Delivery
DMARC Reporting: Analyzing Aggregate and Forensic Reports Email Delivery
IP Warming for Email: Building Sender Reputation from Scratch Email Delivery
Email Feedback Loops (FBL): Processing Spam Complaints Email Delivery
Transactional Email Best Practices: Receipts, Alerts, and Password Resets Email Delivery
DNS Record Types Explained: A, AAAA, CNAME, MX, TXT, and Beyond DNS & Networking
TCP Three-Way Handshake and Connection Lifecycle DNS & Networking
RFC 5321: SMTP Protocol Deep Dive Protocol Deep Dives
RFC 959: FTP Protocol Deep Dive Protocol Deep Dives
TLS 1.3: Faster, Simpler, More Secure Protocol Deep Dives

Debugging Scenarios

554 Transaction Failed — Blacklisted IP Advanced
503 Bad Sequence of Commands — AUTH Before STARTTLS Intermediate

Specifications

RFC 5321 Simple Mail Transfer Protocol

Decision Guides

Why Is My Email Bouncing? Which Protocol Should I Use?

Glossary

Status Code Media Type (MIME Type) TLS Handshake TLS Certificate (SSL Certificate) Cipher Suite Authentication TCP/IP Three-Way Handshake Port Number Firewall IPv6 Exponential Backoff Error Code Mapping Poison Message Request-Response Model RFC (Request for Comments) IANA (Internet Assigned Numbers Authority) Message Framing Connection-Oriented Text vs Binary Protocol MX Record PTR Record SMTP Relay SPF, DKIM, and DMARC Bounce Rate (Email) Email Deliverability MTA (Mail Transfer Agent) SMTP Pipelining Greylisting Backscatter Envelope vs Header STARTTLS SMTP Authentication (SMTP AUTH) Open Relay Email Queue MIME Multipart ARC (Authenticated Received Chain) BIMI (Brand Indicators for Message Identification) DSN (Delivery Status Notification) Email Header Injection SMTP Enhanced Status Codes MDN (Message Disposition Notification) MIME Type TLS Versions TLS 1.3 Certificate Authority (CA) Forward Secrecy (PFS) SNI (Server Name Indication) TLS Session Resumption Key Exchange PKI (Public Key Infrastructure)

Frequently Asked Questions

What is SMTP 421 Service Not Available?

The SMTP service is not available and the server is closing the transmission channel. This may be sent as a greeting or during a session if the server needs to shut down.

When do you see SMTP 421 Service Not Available?

When connecting to a mail server that is overloaded, undergoing maintenance, or shutting down. The server cannot handle your request right now.

How to fix SMTP 421 Service Not Available?

Retry the connection after a delay. If the issue persists, check server status and resources (CPU, memory, disk). Rate limiting or greylisting may also trigger this response.