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ProtocolCodes
450
SMTP

450 Mailbox Unavailable

The requested mail action was not taken because the mailbox is temporarily unavailable. This could be due to the mailbox being locked by another process or a temporary policy restriction.

When You See This Code

When the recipient's mailbox exists but is currently locked, busy, or subject to greylisting. The server is asking you to try again later.

How to Fix

Retry delivery after a few minutes. If using greylisting, the second attempt will typically succeed. For persistent failures, check if the recipient's mailbox is full or disabled.

Specification

View RFC / Specification →

Comparisons

SMTP 450 Mailbox Unavailable vs 550 Mailbox Not Found

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 421 Service Not Available 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

421 Service Unavailable — SMTP Connection Refused Intermediate

Specifications

RFC 5321 Simple Mail Transfer Protocol

Decision Guides

Why Is My Email Bouncing?

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)